Link switch for a burst-switching communications system

ABSTRACT

A high-speed link switch for a distributed-control burst-switching communications system. The switch provides fully integrated voice and data services and processing capacity sufficient to support T1 or higher transmission rates. A communications systems may include a plurality of switches interconnected by time-division multiplexed links. In a preferred embodiment, a link switch comprises a central memory coupled with a link-input processor, a link-output processor, a port-input processor, a port-output processor, and a memory manager. The link switch includes switching intelligence for routing a burst through the switch toward its destination port in the system. A burst is a plurality of bytes which may represent, for example, a block of data or a spurt of voice energy as sensed by silence/voice detectors located at voice ports. Within an outgoing communications link, a burst is transmitted by a switch one byte at a time in an assigned channel of sequential frames of the time-division multiplexed link. The routine intelligence employs dynamic allocation of bursts to channels within communications links such that any channel of a communications link is allocated only when a burst is being transmitted therein and such channel is otherwise available for transmission of another burst.

CROSS-REFERENCES TO RELATED APPLICATIONS

Ser. No. 762,593 now U.S. Pat. No. 4,646,294 Ser. Nos. 762,588, 762,594,now abandoned, 762,591 and 762,590, 762,589, 762,592 now U.S. Pat. No.4,644,529, Ser. No. 762,588 filed concurrently herewith and assigned tothe same assignee hereof, contain related subject matter.

TECHNICAL FIELD

This invention relates to communications switching systems and theircomponents providing integrated voice and data services. Moreparticularly, this invention relates to high-speed switch providingintegrated voice and data services.

BACKGROUND OF THE INVENTION

Communications users, particularly telecommunications users, haverequired ever-increasing ranges of information transport. In thetraditional telephone network, voice signals were transmitted andswitched through the network in analog form. Because of economies incertain types of transmission media, voice signals were digitized fortransmission purposes. Time-division multiplexing of digital voicesignals was the most economical way to utilize the wire-basedtransmission plant of the telephone network.

With the advent of data processing and distributed data processingsystems, a need arose for the transmission of data over communicationslinks and through the telephone network. For purposes herein, "datacommunications" is broadly defined as any information transmittedthrough a digital communications network other than digitized voicesignals. Currently, the most common type of data communications isalphanumerical data, i.e., text or numerical data. Future communicationsrequirements include the ability to carry image and video communicationsin substantial proportions. Image communications is the transmission ofa still picture or motionless object. Facsimile transmission, presentlythe most common form of image communications, is the transmission of theimage of a block or page of information rather than transmission of thedigital representations of the letters or characters which comprise theblock or page. Video transmission adds motion to image transmission. Itcan range from transmission of full motion color television signals tofreeze-frame video, which is a series of sequential still images. Asimage and video communications become more prevalent, the demand forbandwidth will increase dramatically. No doubt, there will be evengreater communications demands in the future, both as to diversity ofservices and traffic capacities.

It is well settled that digital time-division multiplexed transmissionis preferred for both voice and data communications for a number ofreasons not the least of which are the substantial economies realizablefrom digital multiplexing. Digital multiplexing can occur betweencommunications of the same type, such as interleaving a plurality ofvoice conversations onto a single pair of wires. Digital multiplexingcan also occur between communications of different types, such asinserting data communications into detectable silence periods in voicecommunications. Such detectable silence periods may occur while oneconversant is listening or in gaps between words or syllables of aspeaker. Digital multiplexing is particularly suited to adapting tovariable bandwidth demands which result from the inherently "bursty"nature of most voice and data communications. Thus, integration of voiceand data is spurred by the substantial economies of digital multiplexingand the growing diversity of services.

A digital communications network or system is said to be "integrated" orto provide "integrated services" if the network or system has thecapacity to transmit voice and data communications through commonequipment and facilities. An attribute of integrated communicationssystems is the use of intelligent processors at various points in thenetwork for control purposes. Control is "distributed" or "dispersed" ifthe overall network control emanates from multiple geographical points,each point using local information or information provided by distantpoints via the network itself. Thus, the intelligence in a distributedcontrol network is dispersed throughout the geographical area beingserved. In particular, a switching decision which needs to be made by alocal processor can be made with information immediately available tothe local processor. In large communications systems, distributedcontrol generally improves efficiency since the intelligence required toroute local traffic is nearby. Distributed control also enhancessurvivability since a local portion of the system, beingself-controlled, will remain operable in the event a distant controlpoint should be out of service.

With the ever-increasing demand for transmission bandwidth, it isaxiomatic that higher bit rates will be employed over communicationslinks in the future. On the Bell System T1-carrier, of which millions ofmiles are already installed, a communications link carries 1.544 millionbits per second. Links with substantially higher bit rates are feasibleeven with current technology. The provision of integrated services overhigh-speed communications links will require new methods, procedures,and protocols governing information transport through the network. Inparticular, additional bandwidth required by the system for routing andadministration, i.e., the "overhead," should be minimized whilepermitting reasonable flexibility within the network to adapt tochanging circumstances. Integrated switching apparatus should be capableof transmitting and routing information at T1 rates and higher, so thatoptimal channel utilization can be achieved.

Communications systems planners, and in particular telecommunicationssystem planners, seek new switching architectures for a high-speedswitch providing fully integrated services. Such a switch should haveadditional features such as low cost, modular construction, ease ofmaintenance, etc. It would advance the art if such a switchingarchitecture were available.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to obviate the deficienciesin the existing art and to make a significant new contribution to thefield of communications systems.

It is an object of the invention to provide a communications systemhaving fully integrated voice and data services.

An object of the invention is to provide a communications systememploying high-speed communications links, such links having bit ratesof T1 or higher.

It is an object of the invention to provide a communications systemhaving highly distributed control and equipment.

An object of the invention is to provide a communications system whereinthe control functions are administered entirely through the transmissionnetwork; where reallocation of control capacity may be achieved entirelythrough the transmission network, flexibly, and with virtually nodisruption of user services; and where in the event of a failure of acontrol processor, the responsibilities of the failed processor may bereassigned to one or more surviving control processors.

Another object of the invention is to provide an integratedcommunications system which makes efficient use of the copper-wireplants of existing telephone networks.

It is an object of the invention to provide methods of informationtransport within a communications system which require minimal routingand administrative overhead while permitting adequate networkflexibility to adapt to changing circumstances.

An object of the invention is to provide an integrated communicationssystem which features low-cost modular components with highly redundantcircuits well suited for implementation in very large scale integrationtechnology.

It is an object of the invention to provide an integrated communicationssystem which has the capability to transport voice communicationswithout subscriber-perceptible distortion or delay except possibly underoverload conditions.

An object of the invention is to provide a communications system havingthe capability within each switching node to allocate bandwidthdynamically, i.e., within the current communications channel, andthereby to maximize bandwidth utilization throughout the system.

It is an object of the invention to provide methods of informationtransport within a communications system which have the capability ofhandling bursty information, i.e., digital messages of varying length,in a highly efficient manner.

An object of the invention is to provide a link switch which may beemployed in a communications system, such link switch having anembodiment which is relatively small and inexpensive whereby it may behighly dispersed geographically and, if desirable, located nearby or onsubscriber premises.

It is an object of the invention to provide a hub switch which may beemployed in a communications system, such hub switch being a high-speedhigh-capacity switch which may be located at points of highconcentration in the system.

An object of the invention is to provide a high-speed switchingprocessor which may be embodied as a component or as several componentsin a link switch and/or hub switch of a communications system.

It is an object of the invention to provide a high-speed queue sequencerwhich may be employed in some embodiments of a communications system asa component in a link switch and/or hub switch.

An object of the invention is to provide a communications system havingthe capability of providing digital communications from origin port todestination port whereby possible noise interference will besubstantially reduced, ease of maintenance improved, and security andprivacy enhanced.

It is an object of the invention to provide an integrated communicationssystem wherein the transmission rates received at the ports for burstscontaining digitized voice are approximately equal to burst transmissionrates over communications links, so that speed buffering of voice burstswithin link switches is not required.

It is another object of the invention to provide an intelligent portcircuit for a link switch, such port circuit having a high degree ofcontrol intelligence whereby the distributed control feature of acommunications system may be enhanced when the port circuit is remotelylocated.

An object of the invention is to provide a port circuit for a linkswitch which may be located in the vicinity of the subscriber, on thesubscriber's premises, or within end-user equipment, such that call ormessage propagation capacity exists at the hub-switch level, link-switchlevel, and even at the end-user equipment level, if desired.

It is another object of the invention to provide a port circuit for alink switch, such port circuit having a loop-back testing capability,whereby components of a burst switching system may be remotely monitoredfor operability including components of the port circuit itself.

An object of the invention is to provide a method of call set-up andtake-down in a telephone communications system.

It is another object of the invention to provide a highly distributedcontrol architecture for a communications system in which controlcapacity can be added or deleted incrementally with virtually nodisruption in user services.

These objects are accomplished, in one aspect of the invention, by theprovision of a link switch for a burst-switching communications system.A burst is a plurality of bytes including the destination-port address,an information portion, and a termination portion at the end of theburst. The system includes a plurality of switches interconnected bytime-divided communications links. Each communications link has aplurality of frames within each second of time; each frame has aplurality of channels,; each channel has communications capacity for thetransmission of one byte. A byte is a fixed-length string of bits, a bitbeing one binary digit. In a preferred embodiment of the invention, abyte is eight bits, e.g., as with an ASCII character.

The link switch comprises a central memory and link-input processingmeans coupled with the central memory. The link-input processing meanshave means for coupling with a communications link, means for receivingbursts on the communications link, and means for accessing the centralmemory.

Link-output processing means are coupled with the central memory. Thelink-output processing means have means for coupling with acommunications link, means for transmitting bursts on the communicationslink, and means for accessing the central memory.

The switch includes at least one port, each port providing means forcoupling with a control processor, and end-user instrument, or anothercommunications system, said coupling being made via a port-interfacecircuit.

Port-input processing means are coupled between the central memory andat least one of the ports. The port-input processing means have meansfor receiving bursts from the port(s) and means for accessing thecentral memory.

Port-output processing means are coupled between the central memory andat least one of the ports. The port-output processing means have meansfor transmitting bursts to the port(s) and means for accessing thecentral memory.

Memory-management means are coupled with the central memory for insuringthat at any given time only a single access to the central memory mayoccur.

The switch includes means for routing a burst through the switch, whichmay be divided into four types as follows:

(i) means for routing a burst through the switch from an origin port ofthe switch to a destination port of the same switch;

(ii) means for routing a burst through a switch from an origin port ofthe switch to a link of the switch leading toward the destination port;

(iii) means for routing a burst through the switch from a link leadinginto the switch to a destination port of the switch; and

(iv) means for routing a burst through the switch from a link leadinginto the switch to a link of the switch leading toward the destinationport.

The means for routing from port to link and from link to link employdynamic allocation of bursts to channels within links such that anychannel is allocated only when a burst is being transmitted therein andthe channel is otherwise available for transmission of another burst.The switch after receiving the first byte of a burst and also afterdetermining that the burst will be transmitted on a link of the switchleading toward the destination port then transmits the first byte of theburst in the first available channel of a first frame of the linkleading towards the destination port and assigns the channel to theburst. The switch after receiving the second or a successive byte of theburst then transmits the second or successive byte in the assignedchannel of the second or successive frame of the link. The switch aftertransmitting the last byte of the burst then releases the assignedchannel for possible allocation to another burst in the frameimmediately following the transmission of the last byte of the burst.

In an alternate embodiment of the invention, the system comprises a linkswitch having a plurality of ports, so that there is no requirement fora communications link being coupled with the link switch and,consequently, no requirement for link-input processing means norlink-output processing means.

The ports of the link switch may be sequentially coupled on a port bus.In another embodiment, the switch includes a multiplexer circuit, andthe ports of the switch are coupled with the multiplexer circuit.

Thus, there is provided a highly efficient and fully integrated linkswitch which will meet the growing communications needs of the presentand foreseeable future. This switch incorporates many features andadvantages which will be explained in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a burst-switching system.

FIG. 2 shows a preferred embodiment of the digital format of a burst.

FIG. 3 is a block diagram of a link switch in accordance with theinvention.

FIG. 3A illustrates the four types of bursts in transit processed by alink switch.

FIG. 3B illustrates a typical prior art parallel priority-resolvingcircuit which may be used in various embodiments of the invention.

FIG. 4 is a block diagram of a hub switch showing, in particular, thecoupling between the switching units of the hub switch with link groups.

FIG. 5 is a block diagram of a hub switch in accordance with theinvention.

FIG. 6 is a block diagram of an alternate embodiment of a link switchshowing a digital multiplexer coupled between the input and output portprocessors and twenty-four end-user instruments.

FIG. 7 shows the format of a dynamic buffer containing a portion of aburst in the central memory of a link switch.

FIG. 8 diagrammatically illustrates the linkages between buffers forthree bursts on a queue within a link switch.

FIGS. 9A and 9B each show a buffer within character memory of a linkswitch at two different times in the processing of a burst through thelink switch in order to illustrate the input and output indices of thebuffer.

FIG. 10 illustrates the flow of four bursts through the input and outputprocessors and character memory of a link switch.

FIGS. 11A through 11E show the linkages between the input and outputprocessors and the queues and buffers in the central memory of a linkswitch for the various stages in the processing of a burst through alink switch from the time of arrival of the first byte to the time oftransmission of the last byte.

FIGS. 12A and 12B illustrate the assignments of bursts to outputchannels within a link switch in the presence of contention for outputchannels.

FIG. 13 is a pictorial showing a preferred format of a burst includingparticular fields within the four header bytes.

FIG. 14 is a table summarizing the data-link escape procedure inaccordance with the invention.

FIG. 15 is a schematic representation of a hub switch employed in aburst-switching network.

FIG. 16 is a schematic representation of a single switching unit of thehub switch of FIG. 15.

FIG. 17 shows a block diagram of a hub switching element of theswitching unit illustrated in FIG. 16.

FIG. 18 is a diagram illustrating the relationships between hub channelsand hub ring circulation periods during a time-division multiplexed hubframe.

FIG. 19 is a diagram illustrating the format of digital burst signalsprocessed by the hub switch.

FIG. 20 is a table summarizing the operations of a switching unit of ahub switch.

FIG. 21 contains a block diagram of a typical link switch showing thequeue sequencer and various embodiments, or firmware variants, of theswitching processor.

FIG. 22 is a block diagram of the architecture of the basic switchingprocessor.

FIG. 23 is a character state diagram for the finite state machine of theswitching processor showing three states.

FIG. 24 is a channel state diagram for the finite state machine of theswitching processor showing eight states.

FIG. 25 is a block diagram of the architecture of a queue sequencer inaccordance with the invention.

FIG. 25A is a block diagram of an interface circuit employinghandshaking logic which, with appropriate adaptation, may be used as anyof the interfaces in the switching processor or queue sequencer.

FIG. 26 is a diagram showing the microcode format of the queuesequencer.

FIG. 27 is a diagram showing the microcode format of the switchingprocessor.

FIG. 28 shows the memory configuration of the queue sequencer.

FIG. 29 shows the memory configuration of the switching processor.

FIG. 30 is a functional flowchart for the input processors of a linkswitch.

FIG. 31 is a functional flowchart for the output processors of a linkswitch.

FIG. 32 is a block diagram of a port circuit for an analog line whichmay be employed as a component of a link switch as shown in FIGS. 3 and6.

FIG. 33 is a block diagram illustrating service sets and the hierarchyof service providers in a typical control architecture for aburst-switching system.

FIG. 34 is a diagram outlining the steps executed by various controlprocessors required to set up a simple call in a burst-switchingtelephone communications system, such call originating at port X andterminating at port Y of the system.

FIG. 35 illustrates certain control bursts transmitted between controlprocessors in a typical burst-switching control architecture, theillustrated control bursts corresponding to steps in a method of callset-up and take-down in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

Burst switching employs novel methods and equipment for switchingdigitized voice and data in a fully integrated way. As will be evidentfrom the definition of a burst, any form of digital communications maybe handled by burst switching. Burst switching systems typically arecharacterized by highly dispersed small switches, distributed control,and improved bandwidth efficiencies.

FIG. 1 shows a preferred embodiment of burst switching system 100.System 100 comprises high-capacity hub switch 102 and a plurality oflink switches 104. Link switches 104 typically are small switchingelements serving, for example, thirty-two ports or less. Twenty-fourports is a preferred number because of the twenty-four channels in a T1span. Switches are coupled to each other by time-division multiplexedcommunications links 106, e.g., a T1-span. A plurality of end-userinstruments 108 may be coupled via lines 110 with line circuits (notshown in the drawing). The line circuits are in turn coupled with portswhich are components of link switches 104. A port of a link switchprovides means of access to system 100 by users, by control processors,or by other distinct communications systems. A port-interface circuitprovides an appropriate interface with such user, control processor, orother communications system. When interfacing with an end-userinstrument, the port-interface circuit will be denoted herein as a linecircuit. When interfacing with another communications system, theport-interface circuit will be denoted herein as a trunk circuit. Wheninterfacing with a control processor or when speaking generically, theterms "port-interface circuit" or "port circuit" will be used herein.

For purposes herein, T-carriers comprise a hierarchy of digitaltransmission systems designed to carry voice or speech and other signalsin digital form, employing pulse-code-modulation (PCM) and time-divisionmultiplexing (TDM) techniques. The T1-carrier has twenty-four PCM speechchannels. Each signal is sampled 8,000 times per second. Each sample isrepresented by an eight-bit code. Each frame is 193 bits, comprising asample for each of the twenty-four speech channels followed at the endof the frame by one frame-synchronization bit. The T1 line rate is 1.544million bits per second. The T2-carrier has a 6.312 megabit line rateand carries 96 PCM voice channels or the equivalent. These definitionsof T-carriers are used only by way of example. These definitions are notcritical to the operation or description of the invention.

Link switches may be organized into link groups. Any link switch withina link group has the ability to communicate with any other link switchwithin the same link group without the communication passing through thehub switch. In FIG. 1, there are four link groups labelled A, B, C, andD, in the drawing. Any communication between link switch 103 and linkswitch 112 necessarily must pass through hub switch 102; consequently,link switch 103 and link switch 112 are in different link groups, i.e.,link groups and D respectively. Hub switch 102 interconnects distinctlink groups.

A small burst-switching system may not require a hub switch. Forexample, link group A may function as a complete system without hubswitch 102. On the other hand, a large burst-switching system or asystem having high survivability requirements might require more thanone hub switch.

There are numerous alternate configurations for system 100, such asstar, ring, tree-type configurations, and combinations of thesestructures, each configuration having certain advantages anddisadvantages which may be more or less relevant depending on therequirements of the particular application. System 100 as shown in FIG.1 is a preferred embodiment of a burst switch for reasons that are setforth below.

System 100 corresponds to a present-day central office or private branchexchange (PBX) typically having a capacity for servicing 98,000 linesand trunks. In burst switching, the switching function has beendispersed, that is, brought closer to the user. The link switches aresmall so that they can be dispersed into the user neighborhoods orbusinesses. A typical link switch may be mounted on a pole inresidential areas, much like a small distribution transformer, orwall-mounted in a closet in commercial buildings.

The high dispersal of the switching function exhibited in FIG. 1satisfies an objective of burst switching. The telephone industry has alarge investment in outside copper plant. The outside plant represents asignificant replacement value compared to the switching and terminalequipment. Burst switching will extend the usefulness of this plant byexpanding the use of the plant to new services and improving theefficiency of established services.

The Integrated Services Digital Network (ISDN) is one new service area.This network might call for a bandwidth of 288,000 bits/second at thesubscriber instrument or terminal. A short loop leading to a link switchperhaps a few hundred yards away will be better able to support thisbandwidth than will a two-wire pair one to three miles long leading to acentral office.

Burst switching is expected to permit the extraction of new services andbandwidths from the existing plant by moving much of the switching intothe plant. As a side effect, the much shortened average loop length mayshow much less variation in impedance, permitting the striking of bettercompromises in the hybrid network. While there may be some delay througha burst network, burst switching will not require echo suppressors.

For installations in a new subdivision or building, or for replacementinstallations, the amount of copper required to be installed forburst-switch loops is much less than that required for a traditionalswitch. A recent applications study done by GTE Laboratories shows thatburst switches installed in a rural area of about 2000 lines would haverequired only 15% of the outside plant that the present centralizedinstallation required.

In burst switching, data characters are switched in the same way asdigitized voice characters and through the same circuits. Likewise, anytype of digitized analog signals, e.g., image packets, will be handledin a fully integrated way by a burst switching system. Fully integratedswitches will better adapt to changing traffic mixes, from moment tomoment and over the years.

Should the percentage of data compared to voice increase in the comingyears, a burst switch will adapt without reconfiguration since itswitches data in the same way as voice. Burst switching doesdifferentiate between data and voice in one particular. Voice samplesare perishable; excessive delay renders them useless. Data messages canbe delayed much longer with acceptable performance. On the other hand,voice is redundant. Some voice samples may be lost without loss of voicequality. Data is not redundant and no bytes may be lost.

Burst switching therefore switches voice samples at higher priority thandata so that a voice burst may have the first chance at resources in thecase of contention. This minimizes the loss of voice samples (clipping).Data characters will be buffered in the case of contention so that nodata is lost, although delivery may be delayed.

Another instance of integration in burst switching is that any port maybe coupled with a line or a trunk, as well as being either a voice ordata line. The port circuit will differ, but a trunk may appear anywherein the switch. A trunk may be either analog or digital.

A burst may contain a digitized voice (or speech) or data message. Forpurposes herein, a burst containing a communication other than digitizedspeech will be treated as a data communication. A "talkspurt" is asingle continuous emission of voice energy as would occur in thepronunciation of a single syllable. A speaker in ordinary conversationissues voice energy 35-40% of the time. Burst switching employs silence(or speech) detection so that system resources such as transmissionchannels are dedicated only while there is information to send, that is,during the burst. At the conclusion of a burst, the channel is availablefor assignment to another burst. Thus, burst switching utilizes itsresources with two to three times the efficiency of traditional circuitswitching, which dedicates a channel to a conversation for the entireduration of the call.

FIG. 2 shows a preferred embodiment of the digital representation of aburst. Burst 120 begins with a four-byte header, indicated as B₁, B₂,B₃, and B₄ in the drawing. In this burst-switching embodiment and forall purposes herein, a byte is eight bits, although this assumption isnot critical to the invention. In other embodiments, a byte may compriseone bit, or four bits, or any predetermined number of bits. The choiceof eight bits in this embodiment has been made because of the eight-bitcapacity of a T-carrier channel and also because a printable characteris typically represented by an eight-bit code, e.g., an ASCII code. Withthe definition of a byte being eight bits, the term "character" hereinmay be used interchangeably with byte.

In burst 120, the header contains the network address of the burst'sdestination. As a burst enters a switch, the header is interpreted so asto choose the appropriate link (or port) from the switch leading towardthe destination port. The header contains information describing theburst as a voice, data, or control burst. As will be explained ingreater detail below, control bursts are messages exchanged betweenswitch processors. The header information is used to queue bursts foroutput at the desired priority and for other administrative purposes.

The fourth byte of the header is a header checksum which preventsdelivery of a burst to a wrong destination. It is deemed better to abortthe burst and rely on retransmission in the case of data than to deliverthe burst incorrectly. No attempt is made to retransmit voice bursts. Avoice burst's lifetime is so transient that there is only one chance todeliver it correctly. If that chance fails, the burst is too old to beuseful.

Following the header, burst 120 has an information portion comprising N(any integral number) of bytes. Burst 100 ends with a single bursttermination byte, labelled as T in the drawing. Alternatively, more thana single termination byte may be employed as will be explained below.The burst terminator may also be called FLAG herein. Thus, there arefive overhead characters for each burst: the four header characters plusthe terminator.

When FLAG is received, the receiver knows the burst has been completed.Bursts can be of any length, up to continuous transmission. FLAG mayalso used as the channel idle character, so that FLAGs will be sent inan idle channel until the next burst begins.

The bit combination chosen for FLAG may arise among the characters to besent in the burst. There must be no restrictions in the character setavailable to the data sources. It must be possible to send anycombination of binary data.

A data-link escape or DLE character is used to distinguish between theFLAG character as terminator and the FLAG character bit combination asordinary data. At the source, each FLAG or DLE arising in the data to besent is preceded by an extra DLE. At the destination, a received DLE isdiscarded, and the character following the DLE is received withoutexamination for FLAG or DLE, thereby returning the character stream tothat of the source. A received FLAG not preceded by DLE is interpretedas a burst terminator.

Each inserted DLE delays the actual data. FLAG and DLE should be chosensuch that they occur infrequently in the voice samples or data to besent, so as to introduce DLE-insertion delay as infrequently aspossible. As voice traffic is expected to dominate data in volumethrough the end of the century, preferred values chosen for thesecharacters are the codec (voice a/d and d/a) outputs representing themaximum positive and negative amplitudes of the analog voice signal. Analternate choice would be the minimum positive and negative values. Thischoice has particular merit where the minimum values are (in absolutevalue) below the minimum background noise thresholds. In any event, bitconfigurations which represent printable characters should not bechosen, since printable characters likely will occur in high frequenciesin data and text transmissions.

In FIG. 1, communications links 106 between switches are T1 spans,although other rates could be used, for example, T2 or higher. Burstsare sent between switches in time-division multiplexed channels of thespan, with succeeding characters of the burst being transmitted insucceeding frames of the span. The character rate within the channel fora T1-carrier is 8000 characters/second, which matches the codeccharacter generation rate. In burst switching, the voice charactertransmission rate over communication links is matched to the characterrate for the speech character sources and receivers. Therefore, no speedbuffering is required at a link switch for voice bursts although, aswill be explained, buffering is present in the event of contention.Burst output can begin from a link switch as soon as header routing hasbeen done, so the delay through a link switch can be as little as twochannel times, e.g., ten microseconds.

The use of T-carrier channels is an important difference between burstswitching and voice packet switching. In packet switching, a packet istransmitted between nodes using the full bandwidth of the link,transmitting the characters of the packet contiguously. A packet'scharacters will be accumulated at the source (codec) rate, and thentransmitted at a higher rate. This implies that the packet's charactersmust be buffered before the higher rate transmission begins. Since thetime of accumulation introduces a delay, the size of the voice packet isstrictly limited; otherwise, echo becomes a serious problem. A shortpacket means that the header overhead becomes significant. For example,suppose that a voice packet is limited to eight samples, or onemillisecond's worth of data at 8000 characters/second generation rate.If the header overhead is five characters per packet, thirteencharacters must be transmitted to route eight voice samples to thedestination, resulting in a bandwidth efficiency of 8/13 or 62%. If onlythree header characters are required, the voice-packet bandwidthefficiency would be 8/11, or 73%.

By contrast, because the speech transmission rate equals the generationrate in burst switching, there is no need to buffer a speech burstbefore transmission begins. Transmission can begin a character time orso after burst reception begins, and the burst then continues for anyperiod. Header transmission is required only once per burst.

The average length of a talkspurt depends on the silence detectionalgorithm used, but generally exceeds 100 milliseconds. The bandwidthefficiency of burst switching for a 100 millisecond burst is 800/805,which exceeds 99%.

Higher calling rates are expected in the future than those prevailingtoday. Experience has shown that people continue to use their telephonesmore and for more diversified applications. An example is the increasingtransmission of data communications which was virtually nonexistent adecade or two ago.

Another significant example is that of transaction terminals, e.g.,credit card verification terminals, which place calls automatically andhold for only the period of a database access. It is not now possible todetermine how important this expected increase in call placement ratewill be. Nonetheless, an objective of burst switching is to determine anarchitecture of the switch control elements which can grow gracefully tomeet increased calling rates, and which does not exhibit the complexityof multiprocessor central processors.

The traditional approach for common control is to situate thedecision-making element (the common control) at the center of theswitch, to pull in stimuli from the periphery (signaling and supervisionfrom subscribers), and to issue commands for connections to the switchand to the periphery (for example, for ringing).

A more recent control architecture is to move some processing capabilitytoward the network periphery, in the form of slave processors,particularly in remote switching units. These slave processors can dosome lower level preliminary processing, but the final decision makinghas typically remained at the central control point.

The burst switching approach expands this distributed trend to itslimit. The call establishment and feature implementation logic islocated in programmable processors associated with the ports in the linkor hub switches. As the number of ports in a burst network increases, sowill the number of control processors increase, and in a very naturalway. Increased processing capability may be provided by adding aprocessor at an idle port without the complexity and down time entailedby adding a processor to a multiprocessor bus. In a burst-switchingsystem, the number of bursts that can be propagated simultaneously islimited only by the number of port processors and the number of channelsin the system. A port processor is a processor component of a portcircuit which will be explained in greater detail below.

Messages are exchanged between control processors. For example, messageexchange between the calling party processor and the called partyprocessor is required to set up an ordinary call. This message trafficis carried as bursts in the ordinary way and thus adds to the totaltraffic carried by the network, although the message traffic is notsignificant. A three minute call between two parties, each issuing voiceenergy 35% of the time, will generate over one million voice samples;the control message exchange required to set up and release the callwill require less than 100 characters, only 0.01% of the load.

External message exchange is not necessarily required. A port-to-portcall on one link switch can be established entirely within the linkswitch, requiring no external control message traffic. The dispersedprocessors in the link switches also permit continued operation within aburst network region, even if outage prevents the region fromcommunicating with the rest of the network.

Although the decision-making control has been moved to the periphery inburst switching systems, some semi-centralized service processorfunctions remain. In particular, the translation between directorynumber and equipment number (dialed number to burst-switch address) isperformed by a few database lookup machines distributed throughout theburst network. Another similar network function is the recording ofusage information for billing at the conclusion of a call. As will bediscussed below, these administrative processes may be implemented incall processors and/or administrative processors. Either of thesecontrol processors may be coupled with an idle port of the system.

The advantages seen for distributed control are: processor overload iseliminated as a concern; processing capacity is added naturally andeasily as ports are added; and no processor failure, central orotherwise, can bring the burst switching network down.

FIG. 3 shows a block diagram of a preferred embodiment of link switch132. Link switch 132 is coupled between link switches 130 and 134 ofsystem 100 as shown in FIG. 1. In such an arrangement, link switch 132processes four types of bursts in transit as follows: link-to-link orthrough traffic, i.e., bursts passing through link switch 132 from anincoming link to an outgoing link; link-to-port or terminating traffic,i.e., bursts arriving on an incoming link and terminating at a portlocal to link switch 132; port-to-link or originating traffic, i.e.,bursts originating at a port local to link switch 132 and leaving on anoutgoing link; and port-to-port or intra-link switch traffic, i.e.,bursts originating and terminating at ports local to link switch 132.The dashed arrows in FIG. 3A illustrate the four transit types of burstspassing through a link switch. These four transit types are alsodescribed in FIG. 10.

In FIG. 3, link switch 132 includes central memory 160 which is coupledwith six high-speed processors as follows: link input processor (LIP)161, which handles communications incoming from link switch 130; linkoutput processor (LOP) 162, which handles communications outgoing tolink switch 130; LIP 164, which handles communications incoming fromlink switch 134; LOP 166, which handles communications outgoing to linkswitch 134; port input processor (PIP) 168, which handles communicationsincoming from the twenty-four port circuits 178; and port outputprocessor (POP) 170, which handles communications outgoing to thetwenty-four port circuits 178. Each of these processors is a high-speedspecialized switching processor adapted to handle characters andbuffers. As will be explained in detail below, the same high-speedprocessor with slightly different software or firmware may be adapted tomeet the six link-switch processor functions. Central memory 160, havinghigh-speed direct-access memory, is coupled with memory arbiter 172 sothat only one processor of link switch 132 may obtain access at onetime. Memory 160 is the only means of communication between the variousprocessors.

The term "port" does not include port circuit 178. Port circuit 178physically may be located adjacent to link switch 132 or it may belocated remotely, such as on the user premises or within the end-userinstrument. Port circuit 178 may be a line circuit, when link switch 132is coupled with a end-user instrument; or it may be a trunk circuit,when link switch 132 is coupled with another communications system. Aport circuit of link switch 132 may also be coupled with a callprocessor or administrative processor for control purposes, e.g., callset-up. Thus, a port of a link switch, as used herein, describes meanswithin the link switch for coupling with an external circuit or device,but the external circuit or device is not part of the port.

Most of memory 160 is divided into dynamic buffers, which may beassigned to active channels on communication links or ports. Charactersare stored on input in a dynamic buffer assigned for the burst to theinput channel, and characters for output are read from the bufferassigned to the output channel.

The dynamic buffers are employed as ring or circular buffers, so thatthe last storage location in a buffer is considered the predecessor ofthe first. Concurrent input and output can occur, with a character beingread for output only a few character times after it has been stored asinput. Only a few characters will be in the buffer of a burst which isbeing outputted at the same rate it is being inputted. The bufferstorage locations will be used in round-robin fashion, with the output"chasing" the input and a few characters behind.

Although the normal case is concurrent input and output with only a fewcharacters in a buffer, the dynamic buffering technique readily supportschaining buffers together when more than the storage space of a singlebuffer is required. A chain of buffers is formed when each buffer holdsthe address of the next buffer in the chain. This chain structure wouldbe used, for example, when a data burst is temporarily blocked fromoutput because of contention, and the burst's characters are buffered ina chain of one or more buffers until output can begin.

Bursts ready for output are placed on a queue associated with theappropriate output communications link or port. The queues aremaintained in memory 160. A queue entry includes two pieces of data: theaddress of the first buffer of the first burst on the queue, and theaddress of the first buffer of the last burst on the queue. The queuecontains references to bursts awaiting assignment to an available outputchannel.

There are three queues associated with each output link: high, normal,and low priority. Control bursts are queued on the high priority queue,because it is desirable to expedite control bursts through the switchnetwork and because they typically consist of only ten or twenty bytes,thus occupying channels briefly. Voice bursts are handled at normalpriority, and data bursts at low priority. Voice is given preferenceover data because voice samples, if much delayed, become useless. Databursts can be buffered and delayed and still remain within acceptableperformance limits.

In output link channel processing, whenever an idle channel isencountered, the link queues are examined. If there is a burst on atleast one of the queues, the highest priority burst is removed from itsqueue, assigned to the channel, and the first character (or byte) of theburst transmitted. Consider the case of a burst placed on an output linkqueue when it is the only burst queued on the link. The first idlechannel occurring after the burst has been queued will assume itstransmission, thus minimizing interchannel delay. Generally, thetransmission channel will differ from the channel of reception.

The six processors of link switch 132 compete for access to memory 160.When a processor is placing a burst on a queue, for example, more thanone memory access may be required without interruption before the memorycan be made available for use by another processor. Otherwise, the queuereference to the burst may be incomplete. Since all communicationspassing through link switch 132 must pass through memory 160, the speedof link switch 132 depends on the speed of memory 160. For thesereasons, memory 160 is under control of memory arbiter 172.

Memory arbitration means are known in the art. FIG. 3B shows a prior artparallel priority-resolving circuit 450, taken from Y. Pakir,Multi-Microprocessor Systems, page 91, published by Academic Press,1983, which would be suitable with appropriate modification for arbiter172 of FIG. 3. When requests for memory access are pending from severalprocessors, the highest priority request is serviced first by circuit450. Priority may be determined by category and time of request suchthat requests within higher-ranked categories are serviced first andrequests within the same category are serviced on a first-comefirst-serve basis. If only one request is pending, it is servicedimmediately. See also The Handbook of Computers and Computing, edited byArthur H. Seidman and Ivan Flores, pp. 227-232, and further referencescited on p. 232, published by Van Nostrand Reinhold Company Inc., 1984.

Memory 160 includes a timing control, to generate read and write pulses,and random access memory. The buffer address and character index (whichare sent via a character memory bus by a switching processor) areconcatenated to form the address of a unique character.

A burst progresses through a link switch, from input link to outputlink, in the following steps:

1. Incoming communications:

(a) The first byte of a burst is received from an unassigned link inputchannel; the byte is stored in a buffer in memory.

(b) If the first byte contains sufficient information for routing, thebuffer is placed on the appropriate link output queue.

(c) The second byte is received and stored. If the burst has not beenrouted on the first byte and the second byte contains sufficientinformation for routing, the buffer is placed on the appropriate linkoutput queue.

(d) The third byte is received and stored. If the burst has not beenrouted as yet, it is destined for a port on the same link switch. Thethird byte identifies the local port.

(e) The fourth byte is received and stored, and the header checksumcalculated.

(f) If the checksum is bad, receipt of the burst is aborted, andsubsequent bytes before the end-of-burst FLAG are discarded.

(g) If the checksum is good, received bytes are stored in the bufferuntil the end-of-burst FLAG has been received.

2. Channel Congestion:

(a) Channel congestion occurs when there are more bursts on a link'soutput queues than there are idle channels on the output link. Thesystem should be engineered so that channel congestion is an infrequentoccurrence.

(b) While a burst awaits assignment to an idle output channel, inputcontinues to the buffer.

(c) Voice: If two milliseconds' worth of voice samples have beenaccumulated and output has not begun, some or all of the accumulatedcharacters are discarded. This is called clipping. The threshold valueis variable.

(d) Data: If the buffer accumulating data characters fills, anotherbuffer is acquired and linked to the first buffer. Data is not discardedas a result of channel congestion, although its transmission may bedelayed.

3. Outgoing Communications:

(a) When an idle link output channel occurs, the output processor willremove the first burst from the highest priority nonempty queue, andoutput the first byte of the burst.

(b) Thereafter in each successive channel time, the next (successive)byte of the burst is outputted in the same output channel. Input andoutput proceed concurrently.

(c) When the ending FLAG is taken from the buffer, it is sent in thechannel, the buffer returned to the free list, and the channel returnedto idle. The channel is now available for transmission of another burst.

The hub switch is a high-speed high-capacity burst switch used at pointsof high concentration in a burst switching network. The primary functionof the hub switch is to transmit communications between link groups.FIG. 4 shows hub switch 102 of system 100 having means for coupling withlink groups A, B, C, and D. Four switching units, each shown as SU inthe drawing, are connected in a ring about two hubs, 180 and 182. Hub180 carries a parallel transmission of at least one byte in onedirection, and hub 182 has the same parallel capacity in the oppositedirection.

In the embodiment of FIG. 4, each hub switching unit is coupled with twolink groups. SU 184 is coupled with link switch 192 of link group A viacommunications link 188. SU 184 also is coupled with link switch 190 oflink group B via communications link 186. The advantage of thisconfiguration is that each link group of system 100 is coupled with twoswitching units. In the event a switching unit should fail, the linkgroup will not be isolated because of the alternate communications routethrough the other switching unit.

FIG. 5 shows a block diagram of SU 184 of hub switch 102. As shown inFIG. 4, SU 184 is coupled with link switch 192 of link group A and linkswitch 190 of link group B. In FIG. 5, communications link 188 is shownas input line 198 (to hub switch 102) and output line 200, andcommunications link 186 is shown as input line 194 and output line 196.

The architecture of SU 184 is somewhat analogous to that of link switch132. Central memory 202 is coupled in this embodiment with eighthigh-speed processors as follows: link input processor (LIP) 204, whichhandles communications incoming from link switch 190; link outputprocessor (LOP) 206, which handles communications outgoing to linkswitch 190; LIP 208, which handles communications incoming from linkswitch 192; LOP 210, which handles communications outgoing to linkswitch 192; hub input processor (HIP) 212, which handles communicationsfrom hub switching element (HSE) 220 incoming to memory 202; hub outputprocessor (HOP) 214, which handles communications outgoing from memory202 to HSE 220; HIP 216, which handles communications from HSE 222incoming to memory 202; and HOP 218, which handles communicationsoutgoing from memory 202 to HSE 222. The primary function of the hubswitching element is to place communications on and off the hub. Asshown in FIG. 5, HSE 220 provides service to hub 182 which transmits inone direction, and HSE 222 provides service to hub 180 which transmitsin the opposite direction. Memory 202 is coupled with memory arbiter 224so that only one processor may obtain access at any particular time. InFIG. 5, the architecture of SU 184 to the right of dashed line AAresembles closely that of a link switch, i.e., a central memory coupledwith various special-purpose high-speed processors. The HIP, HOP, andHSE are each the same processor as LIP and LOP with firmware or softwaremodifications.

For example, 256 switching units may be coupled to form a hub ring inthe arrangement shown in FIG. 4. In an alternate embodiment, an evenlarger number of switching units may be coupled in the hub ring. Theremay be two or more hub rings in a hub switch, e.g., hubs 180 and 182,for redundancy and to double the number of available transfer channelswithin the hub switch.

Referring to the embodiment of the hub switch shown in FIG. 5, hubs 180and 182 are time-division multiplexed. There are 32 channels on the hubduring each T1 frame time. Each hub channel is divided into 256 clockticks. Each clock tick advances a word on the hub ring from one SU tothe next SU. Within a hub channel (256 ticks), each SU can send a hubword to any other SU since there are 256 switching units on the hubring. Thus, in each hub channel, 256 origin SU's can send 256 hub wordsto 256 destination SU's. A hub word is the full (parallel) complement ofbits that can be transferred simultaneously on the hub.

Assuming the hub is one byte wide, the hub ring clock rate is (8000frames/second)×(32 channels/frame)×(256 ticks/channel)=65,536,000ticks/second. A four-byte-wide hub would require one-fourth this rate,or 16,384,000 ticks/second. Whatever the width of the hub, there is oneadditional control bit, called the "busy" bit.

Each hub signal line has but one load, that of the corresponding bit inthe successor HSE. Higher rates can be sustained in this configurationthan would be possible in a high fan-out bus structure.

As shown in FIG. 4, each SU typically has two T1 links leading to remotelink switches. A hub switch with 256 SU's has the following transfercapacity: (256 SU's)×(2 T1 links/SU)×(24 T1 channels/link)×(8000bytes/second/channel)=98,304,000 bytes/second. As was shown above, a hubring having a width of one byte (so that a byte is advanced on the hubeach clock tick), has capacity of 65,536,000 ticks/second or anequivalent number of bytes/second. Therefore, two hub rings are requiredto support the example of a hub switch having 256 switching units.

Hub switch 184 shown in FIG. 5 is a preferred embodiment, but alternateembodiments are possible and may be desirable to meet specialrequirements of a communications system.

As will be explained below, in order to send a burst from an origin HSEto a destination HSE requires that a hub channel be chosen in which theorigin HSE is transmit idle and the destination HSE is receive idle.Thereafter, the remainder of the burst is sent in that channel. Aburst's progress through a hub switch is similar to a burst's progressthrough a link switch.

1. At the origin HSE:

(a) The burst begins to arrive from an input link.

(b) The burst's characters are buffered in the central memory of theswitching unit. The address of the destination HSE is determined fromthe header of the burst.

(c) The burst is queued for transmission on one hub or the other.

(d) An idle channel is chosen on the hub.

(e) Successive bytes of the burst are transmitted on the hub insuccessive occurrences of the selected hub channel.

2. At each intermediary HSE:

(a) The bytes of the burst are transferred along the hub through the HSEdirectly without passing through the central memory of the switchingunit.

3. At the destination HSE:

(a) The bytes of the burst are taken off the hub by the HSE and storedin the central memory of the switching unit as the bytes arrive.

(b) The header bytes are interpreted to determine the appropriate outputlink.

(e) The burst is queued on the appropriate output link.

(f) Output commences in the first idle output link channel.

In preferred burst-switching network configurations, each link switchhas at least two communications links leading toward any other linkswitch. Either link may be used if the route lengths are similar; thealternate link provides a route in the case of outage. An autonomouscontrol processor coupled with a link switch will permit continuance ofservice among all link switches which can communicate.

The hub switch preferably comprises two load-sharing hub rings, eitherof which could be used to propagate a burst. In the case of hub ringfailure, the other hub ring can be used for any burst. As was shownabove, a single hub ring may not have sufficient capacity to handle peaktraffic. Very large scale integration (VLSI) versions of the HSE's areexpected to be small enough so that an entire hub ring will fit on acard. In this case, the hub switch may comprise two sets of two-hubrings; one ring of each set may be active at all times and the secondring of each set may be backup capacity.

Another alternative is to construct a burst-switch network with morethan one hub switch, so complete failure of a hub switch (requiring twoor more simultaneous failures in the hub switch) would inhibitcommunications in only part of the network. This alternative isattractive in applications requiring a high degree of survivability suchas in military applications where communications outages can resultbecause of hostile action as well as random circuit failure.

Dispersal of link switches makes repair more difficult than repair at acentralized switch. The burst-switching network should have substantialdiagnostic capability so that failures may be identified and remedialaction taken quickly.

A burst-switching network should have background tests and exerciseroutines which run automatically at other than peak-load times. Forexample, adjacent link switches may exchange test messages at regularintervals. A link switch which has not received an expected test messagewithin a prescribed time will make a report via an alternate link switchto a service processor.

Since loops have no appearance at the central office, a link switchshould also have the capability to execute loop and subscriberinstrument tests, automatically or upon control from a mannedmaintenance position, and to return a report of the results.

In addition to the system structure shown in FIG. 1, there are anynumber of alternate embodiments of burst-switching systems. Thefollowing are examples of alternate system structures and embodiments.These examples are intended to be representative but not inclusive. Aburst-switching system may comprise a single link switch providingservice to a plurality of ports. A burst-switching system may comprise aplurality of link switches in a single link group. A burst-switchingsystem may have a plurality of link groups coupled by a hub switchhaving a single hub, or the hub switch may have two or more hubs. Aburst-switching system may comprise a plurality of link groupsinterconnected by a plurality of hub switches, each hub switch havingone or more hubs. In these systems, each link switch provides service toone or more ports. A port may be coupled with a user end instrument viaa line circuit, or it may be coupled with a trunk leading to anothercommunications system via a trunk circuit, or a port may be coupled witha call processor or an administrative processor. As has been mentioned,burst switches are coupled by means of communications links in variousconfigurations, examples being star, ring, tree configurations, andcombinations thereof. Communications links may be full-duplex T-carrierspans.

In FIG. 1, suppose user X of link group A has made a connection withuser Y of link group B. The connection might be called a "virtual"connection since it consists only of X's knowledge of Y's address, andconversely. No resources of system 100 are used except when a burst isin transit.

Assuming X and Y are coupled with voice or speech ports, the followingsummarizes the transit of a burst from X to Y through system 100.

1. When X's voice detector (located in the port circuit) senses voice,it causes a burst header to be issued to X's link switch. The headercontains Y's address.

2. X's link switch determines from the header that the burst must berouted to the hub switch. X's link switch selects the first free channelin a communications link leading toward the hub switch and transmits thefirst byte of the burst in it.

3. Each link switch nearer the hub switch in turn executes the sameprocedure, interpreting the header address and transmitting the burst inthe first free channel of a communications link leading toward the hubswitch.

4. The hub switch determines from the burst header which link groupcontains port Y. The hub switch passes the burst through the hub to Y'slink group.

5. Each link switch in Y's link group interprets the header address andforwards the burst in the first free channel of a communications linkleading toward Y's link switch.

6. Upon receipt of the burst, Y's link switch discards the header, as ithas served its purpose of directing the burst to Y. Y's link switchsends the information portion of the burst to Y.

7. After assigning the outbound channel, each link switch passes theburst through itself, byte by byte, receiving each byte in a channel ofthe inbound link and sending each byte in the channel it has assigned onthe outbound link.

8. Each link switch, when it receives the termination byte of the burst,frees the outbound link channel previously assigned to the burst. Thischannel is now available for assignment to another burst.

Each link switch except the destination link switch makes its routingchoice on the basis of the first header byte alone. The hub switch findsY's link group number in the second byte. Y's link switch finds Y's portnumber in the third byte. Each link switch will delay the burst only acouple of character times as it passes the burst characters along. Ifthe burst could be viewed as it passes from X to Y, one would see theburst strung over all the intermediary link switches, with each linkswitch holding a few characters of the burst.

Each link switch makes it own outbound link channel assignment. Ingeneral, the burst will arrive at a link switch in one inbound linkchannel and leave on a different outbound link channel. If a burstpasses through N (any integral number) communications links between itsorigin and its destination, there will be N independent channelassignments.

If X and Y had been in the same link group, the burst would not havepassed through the hub switch. If X and Y had been on the same linkswitch, the burst would not have passed through a communication link.

LINK SWITCH

Link switch 132 is shown in FIG. 1 with respect to its neighboring linkswitches, link switch 130 on the left and link switch 134 on the right.Every byte of a burst passing through link switch 132 passes throughcentral memory 160, as shown in FIG. 3. The central memory is dividedinto buffers which can be dynamically assigned to a time-divisionmultiplexed channel of a communications link or port. The central memoryis the common and only communication path among the several switchingprocessors. Simultaneous requests for memory access are arbitrated bymemory arbiter 172.

The switching processors administer the movement of characters betweencentral memory and link channels or port circuits. In the embodiment ofFIG. 3, link switch 132 has six switching processors, each of which isbasically the same processor. In different applications within the linkswitch, each processor executes a slightly different program. A controlprogram for each processor is stored in read-only memory (ROM) withinthe respective processor. Each processor has local random-access memory(RAM), in which status and buffer address information is maintained foreach link channel and port served by the processor.

The switching processors are special purpose processors adapted for highspeed. LIP 161, for example, upon receipt of a character in a channel oflink 140, performs all necessary steps for inputting that characterwithin the channel time including the internal administrative stepsrequired for buffer maintenance, channel assignments, etc. LIP 160 mustbe capable of repeating the same steps for a character of another burstarriving in the next channel. Similarly, LOP 162 must be capable ofprocessing and outputting characters at the T1 rate. A T1 channel is5.21 microseconds in duration.

In FIG. 3, PIP 168 and POP 170 use input port bus 174 and output portbus 176, respectively, to scan the port circuits in cyclic fashion, sothat there is a time interval or "channel" associated with each port oneach bus. Thus, the port processors' actions in processing port channelsare similar to the input and output processors' actions in processinglink channels. In the embodiment of FIG. 3, twenty-four port circuitsare serially coupled on port buses 174 and 176. Each port circuit 178performs those functions relating to an individual port, including:burst creation; silence/speech detection for voice ports;analog-to-digital and digital-to-analog conversion for voice ports; andthe required BORSCHT functions, which are the standard functionsassociated with a line card in a traditional digital switching system.

FIG. 6 shows an alternate embodiment of link switch 132 which replacesthe port buses, shown in FIG. 3, between PIP 168, POP 170, and portcircuits 178 with digital multiplexer circuit 250. Multiplexer circuit250 multiplexes between the twenty-four parallel digital lines 256 andT1 path 252 to PIP 168, and between T1 path 254 from POP 170 and thetwenty-four parallel digital lines 256.

This embodiment of a link switch has a number of advantages over theembodiment of FIG. 3. The PIP and POP external interfaces are now thesame T1 interfaces as those of the LIP and LOP, respectively. Portcircuits 258 may now be located at the end-user instruments, e.g.,telephones, providing digital-line interconnects to the link switch withthe benefits of noise immunity and remote testability which digitaltransmission provides.

As has been mentioned, central memory 160 is partitioned into dynamicbuffers. FIG. 7 shows an acceptable format for these buffers. Buffer 300is associated with a burst in transit. For example, the burst may beincoming to the link switch in link channel 1 and outgoing in linkchannel 5. In the local memory of the input processor, buffer 300 willbe associated with channel 1; and in the local memory of the outputprocessor, buffer memory 300 will be associated with channel 5.Therefore, buffer 300 is associated with an input channel (or port) andan output channel (or port).

Buffer 300 contains a fixed number of words, e.g., five words, each wordbeing one byte. It also has three parameters associated with it. Thefirst parameter, NEXT, contains the address of the first buffer of thenext burst on queue. If there is no next burst on queue, NEXT is set toa predetermined character, such as zero. The second parameter, CNT, isthe number of written and unread characters in buffer 300. The thirdparameter, SUCC, is the address of the successor buffer of this burst.If there is no successor buffer, SUCC is set to a predeterminedcharacter, such as zero. The informational portion of buffer 300, INFO1,INFO2, . . . , INFON, (N=5 in this example), contains bytes of the burstin transit. Buffer 300 is shown as containing five informational bytesfor ease of illustration. In a preferred embodiment, buffer 300 contains32 informational bytes.

Normally, only one buffer is required for a burst, and the charactersflow through the buffer from input to output. If a data burst is delayedbecause there are temporarily insufficient link channels, more than onebuffer may be required to hold the burst's characters until an outputchannel can be assigned. In this case, buffers are chained together,with the SUCC field of each buffer holding the address of its successorbuffer of the burst.

A queue is associated with an output communications link or port, andalso associated with a burst-priority type. As shown in FIG. 8, eachqueue includes a queue header and the burst buffers on the queue. Thequeue header comprises two data elements: the address of the firstbuffer of the first burst on the queue, FRST, and the address of thefirst buffer of the last burst on the queue, LAST.

FIG. 8 shows queue 310 having three bursts on it: a first burst of twobuffers with addresses A and B; a second burst of one buffer withaddress C; and a third burst of one buffer with address D. Queue header312 comprises FRST, which contains the address of buffer A, and LAST,which contains the address of buffer D. The arrows in the drawing showthe linkages between the various buffers and queue header.

The informational portions of the buffers are cyclic or ring bufferswherein the last information location in the buffer is treated as thepredecessor of the first informational location. A buffer is used as theexchange medium between an input processor and an output processor. Theinput processor stores a byte of a burst into the buffer using thebuffer address (the memory address of its first location) and an inputoffset from the first location, called PUTINDX. Concurrently, the outputprocessor reads a character of the burst from the buffer using thebuffer address and an output offset from the first location, calledGETINDX. The offset designates or "points" to the location in the bufferinto which the next character of the burst to be received will be storedor from which the next character of the burst will be transmitted.

Both processors use CNT, the count of written but unread characters inthe buffer. CNT is used by the input processor to assure that it doesnot store a character in a full buffer, and it is used by the outputprocessor to assure that it does not read a character from an emptybuffer, "empty" meaning that all characters in the buffer have alreadybeen outputted.

FIGS. 9A and 9B show buffer 320 which, for purposes of illustration, hasfive INFO locations. In FIG. 9A, the input processor has stored thefirst three characters of a burst, "a," "b," and "c." The outputprocessor has not yet begun to output the burst. In FIG. 9B, the outputprocessor has outputted the first three characters of the burst, and theinput processor has stored three additional characters, "d," "e," and"f." The respective positions of PUTINDX and GETINDX are shown in eachdrawing. The horizontal lines through "b" and "c" in FIG. 9B indicatethat these characters have already been outputted although not erased.

FIG. 10 illustrates the flow of four bursts, labelled A, B, C, and D inthe drawing, through link switch 330. Characters of bursts A and B arearriving in two preassigned channels of an inbound link, whilecharacters of bursts C and D are arriving from two local ports. Each ofthe buffers, Buf1 through Buf4, has been assigned to one of the bursts,respectively.

Characters of burst A progress through link switch 330 as follows. Whenthe channel in which burst A is arriving occurs, the LIP receives thenext character of the burst from the channel and deposits it in Buf1.The address of Buf1 is available to the LIP in its local memoryassociated with burst A's input channel number.

When the channel in which burst A is being transmitted occurs, the LOPtakes the next character from Buf1 and sends it in the assigned outboundlink channel. The address of Buf1 is available to the LOP in its localmemory associated with burst A's output channel number.

The characters of bursts B, C, and D are processed in similar fashion.These four bursts represent the four combinations within a link switchbetween links and ports. Burst A passes through link switch 330 frominput link to output link; burst B, from input link to output port;burst C, from input port to output link; and burst D from input port tooutput port.

The continuous flow aspect of a burst through a link switch should benoted. The storing of individual characters in buffers has beendescribed. Over a period of time, a stream of bytes, i.e., a burst,flows through a buffer with straightforward logic and high efficiency.The buffers are dynamically assigned to bursts in transit through thelink switch. This is true even in the case where both the origin anddestination ports are local to the same link switch, e.g., burst D.

In the following, a burst will be traced through a link switch from thetime of arrival of the first byte or character until the last characterhas been sent. FIG. 11A shows the conditions in central memory 340before the first character of the burst arrives. The buffer, labelled"BUF" in the drawing, which will be assigned to the burst is on the freelist, F. The free list is a queue containing the addresses of thosebuffers available for assignment. Arrow 342 indicates that F points toBUF as being available for assignment.

FIG. 11B shows the conditions in central memory 340 after the firstcharacter has arrived. The LIP has removed BUF from F and stored BUF'saddress in the portion of its local memory associated with the inputlink channel. Arrow 346 indicates the association in the LIP's memorybetween the input link channel and BUF. The LIP has stored the characterinto BUF; determined from the burst's destination address in the firstheader character that the burst is to be forwarded via the output link;and placed the burst on an outbound link queue, Q. Q contains referencesto bursts awaiting assignment to a channel for output on a particularcommunications link. Arrow 344 indicates that Q points to BUF as readyfor assignment to an open channel in the output link associated with Q.

FIG. 11C shows the conditions in central memory 340 after the outputlink channel has been assigned. The LOP has found a free output channeland examined Q to see if any bursts are ready for assignment to anavailable output channel. Further, the LOP has found the burst's bufferaddress in Q; removed the burst from Q; stored the buffer address in theportion of its local memory associated with the output channel; and readthe burst's first character from BUF and transmitted it in the outputchannel. Arrow 348 indicates the association in the LOP's memory betweenBUF and the output channel. These LOP actions have been performedindependently of the LIP, except that the two processors havecommunicated via central memory 340 and they may have contended foraccess to the central memory.

The conditions shown in FIG. 11C will prevail for most of the remainderof the burst. As the burst's input channel occurs, the LIP takes thenext character of the burst and stores it into BUF. As the burst'soutput channel occurs, the LOP reads the next character from the bufferand outputs it. The LIP and LOP each know the buffer identity, becauseeach processor has stored the buffer address in its local memory.

In the normal case, the LIP is one character ahead of the LOP so thatBUF contains one character at any time. In the event there is a delay inthe output channel assignment, the LIP will be more than one characterahead of the LOP, and there will be more than one character in thebuffer during the burst except for the transmission of the lastcharacter.

FIG. 11D shows the conditions in central memory 340 after the LIP hasdetected the termination character at the end of the burst. The LIP hasstored the termination character into BUF and dissociated BUF from theinput channel in its local memory. The LIP is ready to begin receivinganother burst in the same input channel which, if present, would bestored into a new buffer in central memory since the LOP may still beusing the old buffer for outputting the first burst. The LOP continuesto output the remaining character or characters in BUF independently ofthe LIP.

FIG. 11E shows the conditions in central memory 340 after the LOP hasfound the termination character in BUF. The LOP has read the terminationsequence from BUF and transmitted it; and returned BUF to the free list.

To expand on the assignment of output channels to bursts in a situationwhere there is contention for output channels, the following example isprovided. FIG. 12A shows link switch 360 coupled with link switch 362via communications link 364 in which there are only two channels (sothat the example will be short). Three users, A, B, and C, desire tosend four bursts over link 364; two bursts originate from user A, andone burst each originates from users B and C. It is assumed that thebursts are in the same priority class.

FIG. 12B is a pictorial of link 364 from link switch 360 to link switch362 in which the assignments of bursts to the two channels as a functionof time are described. As indicated in the drawing, time increasestoward the left so that the rightmost slot is earliest in time.Initially, both channels are idle, as indicated by "X" in both channelslots 366 and 368. At time a, link switch 360 receives the first byte ofthe first burst, A₁, from user A. Link switch 360 transmits the firstcharacter of A₁, in slot 370, which is the first idle output channeloccurring after time a. Slot 370 represents Channel 1. Channel 2continues to be idle, as shown in slot 372.

At time b, link switch 360 receives the first byte of a burst from userB. The first available output slot is Channel 2 in slot 374, and thefirst character of burst B is transmitted therein.

At time c, the first byte of a burst from user C has been received bylink switch 360. Since both channels are assigned, burst C will beaccumulated in a buffer or buffers of the central memory (the length oftime depending on whether burst C is a voice or data burst) and placedon a waiting queue until a channel is available. The bar over A₁ in slot376 indicates the terminator character for burst A₁. Therefore, Channel1 will be free during the succeeding frame. The first character of burstC is transmitted in slot 378.

At time d, the first byte of a second burst, A₂, has been received bylink switch 360. Since both channels are occupied, A₂ is accumulated andplaced on a waiting queue. Burst B terminates in slot 380, and the firstcharacter of burst A₂ is transmitted in slot 382.

In slot 384, burst C terminates. Since there are no unassigned burstsawaiting transmission, Channel 1 becomes idle in slot 386. Likewise,Channel 2 becomes idle in slot 390 after the termination of burst A₂ inslot 388.

This example illustrates queuing of bursts during contention for outputchannels and that channel assignment is only for the duration of aburst. The example further illustrates that user A's first burst wasassigned to Channel 1 and A's second burst was assigned to Channel 2.

Each link switch through which a burst passes forwards the burst towardits destination port on the basis of the destination port's equipmentaddress contained in the burst's header. Referring to FIG. 1, assume aburst originates at port X of link group A and its destination is port Yof link group B. A port address has three components: link group; linkswitch within link group; and port number within link switch.

Each link switch has three priority queues associated with each of itscommunications links, one queue for each type of burst. There are threetypes of bursts: control, voice, and data. Control bursts have highpriority. It is desirable that they propagate quickly through the systemin order to maintain system responsiveness. Since control bursts areshort, they will not occupy channels for long periods. Data bursts havelow priority. Data bursts can accommodate delays better than voicebursts; consequently, this type of burst can be buffered effectively.Voice bursts have middle priority. Voice bursts have preference overdata bursts because voice samples, if delayed substantially, havediminishing value.

FIG. 13 shows format 400, a preferred format of a burst. A burstcomprises a sequence of eight-bit bytes or characters, having afour-byte header preceding a variable-length information portionfollowed by a termination character at the end of the burst. The firstheader word contains three fields: burst type, group, and destinationlink switch, labelled in the drawing as BT, G, and DLS, respectively.The burst type may be 0, 1, or 2, indicating that the burst is acontrol, voice, or data burst, respectively. The group bit may be 0or 1. When G=1, the burst header is currently in a different link groupthan the link group of the destination port. Thus, the burst will berouted through the hub switch. When G=0, the burst header is currentlylocated in the link group of the destination port. Note that the G bitis reset as the burst header passes through the hub switch into thedestination link group. DLS ranges from 0 to 15 and indicates the numberof the destination link switch within the destination link group.

The second header byte contains DLG, the destination link group number,which ranges from 0 to 255. The third byte of the header contains DP,the destination port number, which ranges from 0 to 31. The destinationport number is within the destination link switch, which in turn iswithin the destination link group. The fourth header byte, HCS, containsthe header check (sequence. HCS ranges from 0 to 255 and provides meansfor corroborating error-free receipt of the first three words of theburst header.

Following the burst header is the information portion of the burst. Theinformation portion has a variable number of bytes, meaning that thenumber of information bytes generally will be different in every burst.In control and data bursts, the last two bytes of the informationportion may contain a burst check sequence, indicated as "(BCS)" in thedrawing. The burst check sequence provides means by which the presenceof errors in the received information portion of a burst may bedetected. When an error is detected, the recipient may attempt tocorrect the error with error correction techniques or the recipient mayrequest retransmission of the burst.

The termination character, TC, defines the end of a burst. As will beexplained below, use of a data-link escape character in combination withthe termination character will provide the system with means fordistinguishing the termination character as a data character occurringin the middle of a burst and the termination character as a terminatoroccurring at the end of a burst. Termination characters are transmittedin idle channels to indicate the availability of these channels forassignments to bursts.

There are numerous alternate definitions of burst formats which may beappropriate for various system configurations. Format 400 has beendescribed as an example. Assuming format 400 is employed in aburst-switching system, a link switch routes a particular burst in thefollowing steps.

1. Upon receipt of the first header byte of a burst, the link switchexamines the G bit. If the G bit is set, the link switch is not a memberof the destination link group and the burst must be routed to the hubswitch. The link switch places the burst on the appropriate priorityqueue of the communication link leading toward the hub switch. If the Gbit is reset, the burst is in the destination link group and the DLSfield of the first header byte must be examined to determine whether ornot the link switch is itself the destination link switch. If the linkswitch is not the destination link switch, the burst is placed on theappropriate priority queue of the link leading toward the destinationlink switch. If the link switch is itself the destination link switch,the link switch holds the first header byte awaiting the remainder ofthe header and, in particular, the third header byte in which thedestination port is specified.

2. Upon receipt of the second header byte of a burst by a link switchnot in the destination link group (G bit set), the link switch passesthe second header byte through on the assigned communications linktoward the hub switch. Upon receipt of the second header byte by a linkswitch within the destination link group (G bit reset), the link switchdetermines whether or not it is the destination link switch. If the linkswitch is not the destination link switch, the link switch passes thesecond header byte through on the assigned link to the destination linkswitch. If the link switch is itself the destination link switch, thesecond header byte is held awaiting the third and fourth header bytes.The second header byte is used by the hub switch to route bursts betweenlink groups. In the process of routing a burst into a destination linkgroup, the hub switch resets the G bit in the first header byte so thatlink switches within the destination link group may determine theburst's status from the first header word of the burst. If there aremore than one hub switch, the hub switch which passes the burst into thedestination link group, i.e., the last hub switch through which theburst passes, resets the G bit.

3. Upon receipt of the third header byte of a burst, the action of thelink switch again depends on whether or not the link switch is thedestination link switch. If the link switch is not the destination linkswitch, the link switch passes through the third header byte on theassigned link toward the destination link switch. If the link switch isitself the destination link switch, the link switch determines thedestination port from the DP field of the third header word.

4. Upon receipt of the fourth header byte of a burst, the link switchchecks the header check sequence in the HCS field. If HCS is bad,indicating there is at least one error in the burst header, the linkswitch aborts transmission of the burst by sending the terminationsequence in lieu the remainder of the burst. If HCS is good, the actionof the link switch depends on whether or not the link switch is thedestination link switch. If the link switch is not the destination linkswitch, the link switch passes through the fourth header byte on theassigned link to the destination link switch. If the link switch isitself the destination link switch, the link switch discards the entireburst header and places the burst on a queue to the destination port.

The last character of a burst is always a termination character.Whenever an output channel is idle, termination characters aretransmitted in the channel. When a channel becomes idle after thetransmission of a burst, there will be one or more terminationcharacters following the burst in the channel. This adds a degree ofsafety in the event a termination character is sent at the conclusion ofa burst, but the termination character is not received as such becauseof error.

The termination character has a unique character code. It must bepossible to send any character stream through a link or hub switch,including streams in which the termination character code itself occurs.A method is required by which the termination character can berecognized as an end-of-burst when so intended or as a data character ina stream when so intended.

The method employed for distinguishing the termination character from adata character is derived from the escaping technique of BinarySynchronous Communications. It is similar to the bit-stuffing techniqueof HDLC (High-level Data Link Control) except that in this case, themethod is a byte-stuffing technique. The byte that is stuffed orinserted is a second special character called data-link escapecharacter. In the following, the termination character is designated byTRM; and the data-link escape character, by DLE.

At the source, whenever a TRM or DLE bit configuration arises in data tobe sent, a DLE is inserted before the data character for transmission.Thus, the source transformations are:

TRM is replaced by DLE TRM;

DLE is replaced by DLE DLE;

X is replaced by X, if X does not equal TRM or DLE.

At the destination whenever DLE is received, it is discarded. Thecharacter received immediately following the discarded DLE is acceptedwithout examination for control significance. Thus, the destinationtransformation is:

DLE Y is replaced by Y, Y is any character.

If a TRM is received without a prefixing DLE, the TRM interpreted as anend-of-burst character. FIG. 14 contains Table 410 summarizing the datalink escape procedure.

Each inserted DLE delays the actual characters of the burst.Accordingly, the codes for TRM and DLE must be chosen such that theyoccur infrequently in the voice samples and data to be sent. Since voiceis expected to dominate data in volume within the foreseeable future,meritorious choices for values of these characters are the codec (voiceA/D and D/A) outputs representing the maximum positive and maximumnegative amplitudes of the analog voice signal. As previously discussed,the minimum amplitudes may be alternate choices in appropriatecircumstances.

As has been stated, every burst is terminated with one or more TRMcharacters. Suppose a burst is terminated by a single TRM character.Then, any character in a burst altered by noise into TRM, or any DLE TRMaltered by noise into X TRM, would cause the switch to treat the burstas two distinct bursts. The latter part of the original burst, nowerroneously treated as a second burst, would likely fail the headercheck sequence test, so that the "second" burst would not be deliveredto its intended destination. Conversely, suppose a single TRM betweentwo actual bursts should be altered by noise into a non-TRM character.The second burst, now erroneously treated as part of the first burst,will be erroneously delivered to the destination port of the firstburst.

The probability of these errors can be reduced to any desired smallvalue by requiring that a burst be terminated by a redundant sequence ofTRM characters rather than by a single TRM. For example, the terminationcharacter sequence could comprise five TRM characters, with end-of-burstdeclared upon the reception of three TRM characters within anyfive-character sequence. In order for an end-of-burst error to occur inthis case, three non-TRM characters would have to be changed to TRMcharacters, or three TRM characters would have to be changed to non-TRMcharacters. The probability of error has been reduced substantially overthe case of a single-TRM character procedure.

There are any number of burst termination schemes that could be employedin a burst-switching system. The choice for any particular system willdepend on system characteristics and design goals. The examplesdiscussed herein are intended to illustrate the wide range of optionsavailable.

The link and hub switches described herein have autonomous "in-line"switching capability which by itself distinguishes them from theirexisting art counterparts. Each switch may be connected betweencommunications links having T1 (or higher) capacity. With each incomingchannel, each switch has the capability to make and implement anappropriate routing decision for the information contained within thechannel. The routing decision is made autonomously by the switch withoutreliance on any outside source, e.g., a central control switch.Moreover, all of the processing relating to the routing decision isperformed within the time allotted to the incoming channel. When thechannel time has elapsed, the switch is ready to repeat the procedurefor information contained in the succeeding channel, and so on. Thus,the switch processing is in large part synchronized with the channel andframe timing of the communications links. In some embodiments, theactual channel slot timing is employed as a "restart" signal orinterrupt for commencement of the switching algorithm. As has beenexplained, the switching algorithm must be capable of (among otherfunctions) originating, continuing, and terminating burst transmissionswhich are in transit through the switch.

Burst switching fully integrates voice and data bursts. Generallyspeaking, only one character's worth of buffering is required in burstswitching, because the voice transmission rates are matched with the T1rate. Burst switches move every burst through a dynamic buffer; in theevent of temporary channel contention, information (especially data)will not be lost.

The delay through a burst switch is an important performance parameterfor voice transmission. Too much delay will make echo intolerable. Inburst switching, the characters of a burst generally pass through aswitch with a delay of less than four channel times. No speed bufferingis required and, consequently, neither echo suppressors nor echocancelers are required.

Different bursts on the same call may have different channel-exchangedelays through the switching nodes. However, all characters within aspeech burst experience the same delay. The magnitude of the variabledelay component between talkspurts is less than the average silenceperiod between talkspurts. Therefore, the variable delay betweentalkspurts is virtually unnoticeable.

A burst may be of any length, and a single header suffices for everyburst. A typical talkspurt averages about 250 milliseconds, or 2000pulse-code modulation characters. Assuming a burst header of four bytesand a one-byte terminator, the overhead for each burst is five bytes.The burst header overhead for the average talkspurt expressed as apercentage is 5/2000 or less than one percent. If a five-byte terminatorsequence is employed instead of a one-byte terminator, the burst headeroverhead is still less than one percent.

A burst switch easily handles data transmissions at rates less than 64kilobits per second (kb/s). When data is received from a user at lessthan 64 kb/s, a conveniently sized block of such data is accumulated inthe port processor. This block is then transmitted through the system asa burst at the 64 kb/s rate. The same method would apply where theburst-switching channel rate is other than 64 kb/s.

It is expected that the switching services of the future will require awide variety of bit rates, from low-speed data terminals of, say, 1200bits/second, through digitally encoded voice of 16 to 64kilobits/second, to high-speed data devices and digitally encoded video.The term "bandwidth efficiency" is often used to denote the ease withwhich a switch handles a variety of transmission rates. Because a bursthas message structure as well as channelized operation, transmissionrates higher than the channel rate can be handled with relative ease byemploying a number of channels together for the transmission of a singleburst. In a burst-switching system having a 64 kb/s channel rate, anN×64 kb/s burst would be treated as N separate (but related) bursts eachat the 64 kb/s rate, where N is an integer greater than one. The Nrelated bursts may be transmitted to the burst's destination in separatechannels and then reassembled into the original N×64 kb/s burst. Themessage structure of burst switching permits reassembly of relatedbursts in proper order even though the N related bursts may not arriveat the destination in phase synchronization.

Future switching services are expected to require greater digital datahandling capacity. Burst-switching systems may operate in a link-switchlevel error checking and retransmission mode for data bursts. Each databurst is fully buffered at each switch. The error-check test for a burstmust be passed before the burst is retransmitted to the next switchalong the route. Another error-checking mode is end-to-end errorchecking. In this mode, error checking of a data burst is performed onlyby the destination link switch. If the error check fails, thedestination link switch requests retransmission of the data burst by theorigin link switch.

The notion of end-to-end error checking is extended to the ultimate whenthe error checking is performed in the port processors and the portprocessors are located on user premises or within end-user instruments.The end-to-end digital-transmission capability possible with thisarchitecture yields other advantages, such as improved diagnosticcapability and enhanced security and privacy of voice and datacommunications. In the latter case, encryption and decryption of digitalcommunications can also be performed in the port processors.

HUB SWITCH

The hub switch 500 as illustrated in FIG. 15 is a high-speed,high-capacity TDM switch for transferring the bytes of a message burstreceived in any inbound link channel from any link group to an outboundlink channel of the appropriate link group as determined by addressinformation in the message burst. The hub switch 500 includes Nswitching units 501 connected in a ring. In order to provide theadvantages of redundancy, two hub buses 502 and 503 may be provided topropagate signals around the ring in either direction. Each switchingunit is connected to a link group by one or more TDM link communicationlinks 504.

An origin switching unit which receives a byte of digitally encodedinformation on an incoming link channel places the byte on the ring. Thebyte is passed from switching unit to switching unit around the ringuntil it arrives at its destination switching unit as designated byaddress information contained in the message burst.

As illustrated by the diagram of FIG. 19, a burst consists of a header(HDR), the information or data being transferred (INFO), and atermination character sequence of termination characters (TCS). Theheader contains the address to which the burst is being sent along withother identifying information on the burst. The information portion ofthe burst is a continuous stream of bytes. The length of the burstvaries. Usually for speech information the burst is between 100 and 300milliseconds in length. The termination character sequence oftermination characters (bytes) indicates the end of the burst. Thesequence of termination characters is continuous within a channel whileit remains idle.

Transfer of bytes of digitally encoded information around the hub ringtakes place in C hub channels having the same frame period as theinbound and outbound TDM link channels. Each byte moving around the hubring from an origin switching unit to a destination switching unitduring a hub channel time slot must be transferred from switching unitto switching unit at a rate so as to propagate completely around the hubring in a ring circulation period equal to the hub channel time slot.Movement of a byte from one switching unit to the next takes placeduring each tick of a central clock 505.

As illustrated by the diagram of FIG. 18, during each frame there are Cchannel time slots, and during each hub channel time slot there are Nclock ticks. In the illustrative embodiment under discussion the frametime is 125 microseconds, the same as the T1 frame time of the linkchannel in the link communication links 504. The number of hub channelsper frame, C is 32. C can not be less than the number of link channels(24 in a T1 system). The number of clock ticks in a frame is C×N. In theembodiment under discussion the number of switching units, N, is 256.

In order for communication to take place between the different linkgroups connected to the various switching units of the hub switch, thelink switch of the origin link group must find an idle channel to itsassociated switching unit of the hub switch. This origin switching unitin the hub switch must then find an idle hub channel between itself andthe destination switching unit of the hub switch. Finally, thedestination switching unit must find an idle link channel to communicateto the destination link switch in its associated link group.

Since there may be congestion at the hub switch switching units suchthat an idle hub channel or outgoing link channel is not immediatelyavailable when needed, and since some channel slip between channels isinevitable, each switching unit must include buffer memory andprocessors to manage the memory. FIG. 16 illustrates a switching unit.The switching unit includes a first hub switch element 515 connected tothe hub bus 502 for transferring bytes in one direction around the hubswitch ring. A second hub switch element 517 is connected to the hub bus503 for transferring bytes around the hub switch ring in the oppositedirection. The switching unit also includes a memory 516 and processorsfor managing the information into, out of, and within the memory. Theprocessors, which are designated with respect to the memory, include twolink input processors (LIP) 521 and 526 between incoming link channelsand the memory and link output processors (LOP) 522 and 527 between thememory and the outgoing link channels. A hub input processor (HIP) 523and a hub output processor (HOP) 524 are between the hub switch element515 and the memory 516. The processors associated with the second hubswitch element 517 are a HIP 518 and a HOP 519.

A principal function of the processors is controlling the routing ofbytes between the memory 516 and the hub channels, and between thememory 516 and the link channels. They also have other functionsincluding the acquisition of channels and the assignment anddeassignment of dynamic memory buffers within the memory 516 to hub andlink channels. These and other functions such as sequencing and queuingare managed in essentially the same manner as similar functions areperformed by the link switches described in detail previously. Thefunctions of transferring bytes from the memory 516 to the hub ring 502by way of the hub switch element 515 and transferring bytes off the hubring 502 by way of the hub switch element 515 are controlled by the HOP524 and HIP 523. The memory 516, HIP 523 and HOP 524 together with a LIPand a LOP in effect form a variety of link switch serving as aninterface between the link group and the hub switch element 515. Theprocessors associated with the second hub switch element 517 function ina corresponding manner in transferring bytes between the memory 516 andthe hub ring 503.

In summary, a message burst passes through the hub switch from anincoming channel of one link group to an outgoing channel of anotherlink group in the following manner. Bytes of the burst arriving at theorigin hub switching unit 501 on an incoming link channel are bufferedin the switching unit memory 516. The first bytes, or header, of theburst contain address information; one byte, specifically the secondbyte, designating the destination link group and therefore thedestination switching unit. The received bytes are queued fortransmission on the hub bus. A hub channel in which the origin switchingunit is transmit idle and the destination switching unit is receive idleis selected. The bytes of the burst are loaded onto the selected hubchannel, one byte during each hub channel frame. A byte is transferreddirectly between the hub switch elements of adjacent interveningswitching units on each clock tick without passing through the memories516. Upon arriving at the destination switching unit, each byte isstored in the memory. The header bytes are interpreted to determine theappropriate output link group, if more than one link group is associatedwith the destination switching unit. The bytes are queued on theappropriate outbound link, and output begins on the first idle outboundlink channel.

Hub Channel Transfer-General

FIG. 17 is a block diagram illustrating the first hub switch element 515of a switching unit 501. The hub switch element handles the transfer ofbytes from the preceding hub switch element and to the succeeding hubswitch element along the hub ring bus 502. Also, under control of theHOP and HIP the hub switch element loads bytes from the memory 516 tothe ring when the switching unit is an origin, and unloads bytes fromthe ring into the memory 516 for transmission on an outbound linkchannel when the switching unit is a destination.

Each hub switch element includes a destination memory 540 which containsthe switching unit destination address for each hub channel in which thehub switch element 515 is transmit active. In addition a transmit activememory 559 contains a bit for each hub channel indicating the transmitbusy or idle status of each hub channel for the hub switch element. Eachhub switch element also includes a destination counter 531 which at thestart of each hub channel or ring circulation period is set to the hubswitch element's address. On each clock tick (TCLK) the destinationcounter 531 is decremented. Also on each tick the byte circulating onthe ring which is in the THIS-SU storage register 532 of the hub switchelement is transferred on the hub bus 502 by a multiplexer 533 to thestorage register of the next succeeding switching unit. At the same timethe byte in the register of the preceding switching unit moves into theTHIS-SU register 532 of the switching unit.

When the hub switch element 515 of the switching unit 501 is serving asan origin for a message burst, the byte to be transferred during a hubchannel time slot is placed in the hub-in data register 535 by way of ahub-in data series register 539 at the start of the hub channel timeslot by the HOP to await transfer onto the ring. At the same time, anactivity bit indicating that the hub switch element needs a hub channelis placed in a need channel register 545. Also, the destinationswitching unit address is placed in the destination register 536 fromthe destination memory 540 at the start of the hub channel time slot orring circulation period. Upon the clock tick that causes the contents ofthe destination counter 531 to be the same as the address in thedestination register 536, a comparator 537 produces an output. Thisindication of a match is applied to a channel acquisition and datatransfer section 538 which causes the multiplexer 533 to transfer thecontents of the hub-in data register 535 rather than the contents of theTHIS-SU register 532 onto the hub bus to the succeeding switching unit.

As stated above, each byte on the ring is transferred from one switchingunit to the next succeeding switching unit on each tick during the hubchannel time slot. On the last tick of the ring circulation period atthe end of the hub channel time slot each byte on the ring istransferred to the THIS-SU register 532 of its destination switchingunit. On the next tick starting a ring circulation period and hubchannel, the byte stored in the THIS-SU register 532 is transferred tothe hub-out data register 548 for placing in the memory 516 by the HIPand transmission on an outbound link channel by the LOP.

Hub Channel Acquisition-General

Movement of the bytes of a message burst during successive frames of ahub channel involves coordination between a HOP of the origin switchingunit and a HIP of the destination switching unit. The HOP controlsfetching a byte from the memory of the origin switching unit and placingit on the hub ring bus, and the HIP of the destination switching unittakes the byte from the hub ring bus and places it in its memory. TheHOP can process only one byte movement and the associated functionsduring each hub channel or ring circulation period, and the HIP canprocess only one byte movement and the associated functions during eachhub channel or ring circulation period. Thus, for each burst, a free hubchannel must be found during which both the origin switching unit istransmit idle and the destination switching unit is receive idle.

The need for acquiring a hub channel is recognized by the originswitching unit when an incoming link channel becomes busy. Therefore,the finding of a free hub channel must be accomplished at the originswitching unit. The origin switching unit knows its transmit busy/idlestatus for each of the hub channels. In order to select a free channel,it must also have information on the receive busy/idle status for eachof the hub channels for the destination switching unit.

In order to provide information on the receive busy/idle status of eachswitching unit during the ring circulation period of a hub channel onthe hub ring bus 502, an activity line 541 is provided in the hub ringin parallel with the hub ring bus 502. A receive activity memory 543stores a bit for each hub channel indicating whether the hub switchelement 515 is receive busy or receive idle for that hub channel. At thebeginning of each hub channel period the bit indicating the receivebusy/idle status of the hub switch element for that hub channel istransferred to the THIS-SU register 532 of the succeeding switchingunit. This activity bit is propagated around the ring from switchingunit to switching unit during subsequent ticks. Thus, any switching unitcan determine the hub channel receive busy/idle status for any otherswitching unit by examining the activity bit placed in its THIS-SUregister 532 on the appropriate tick during the ring circulation period.

When an origin switching unit must find a free hub channel to adestination switching unit, the address of the destination switchingunit is placed in the destination register 536 of the hub switch elementand the first byte of the burst which is to be transmitted istransferred into the hub-in data unit register 535 at the start of thefirst hub channel in which the hub switch element of the originswitching unit is transmit idle. In addition the HOP sets the needchannel register 545 to indicate the need for a hub channel fortransmitting the byte in the hub-in data register 535.

On the clock tick when the contents of the destination counter 531 arethe same as the contents of the destination register 536, the comparator537 produces an output indicating that it is the appropriate point inthe ring circulation period for loading the byte in the hub-in dataregister 535 onto the hub ring bus 502. The activity bit in the THIS-SUregister 532 indicates the receive busy/idle status of the destinationswitching unit, and a bit in the transmit activity memory 559 indicatesthe transmit busy/idle status of the hub switch element of the originswitching unit. If these bits indicate that the destination switchingunit is receive idle and the origin switching unit is transmit idle forthis hub channel, a hub channel has been found for sending the burstfrom the origin switching unit to the destination switching unit.

The origin switching unit seizes this hub channel by setting theactivity bit to busy as it is transmitted by the multiplexer 533 to thesucceeding switching unit on line 541. At the same time the first byteof the burst is transferred from the hub-in data register 535 to thesucceeding switching unit on line 502 by the multiplexer 533. Inaddition, the channel acquisition and transfer section 538 sets achannel seized register 546 to indicate to the HOP that a successful hubchannel acquisition and data insertion on the hub ring bus have beenmade. The HOP stores the appropriate information in the memory 516 sothat subsequent bytes of the burst will be transferred into the hub-indata registers 539 and 535 at the appropriate times for transmission tothe destination switching unit on the acquired hub channel duringsuccessive frames. The indication that the acquired hub channel is nowtransmit busy is placed in the transmit activity memory 559 and theaddress of the destination switching unit for the hub channel is placedin the destination memory 540 to complete the hub channel acquisitionprocedure.

Since the activity bit being propagated on the activity line 541 is setto busy by the origin switching unit when seizing a hub channel, anydownstream switching unit which may also be seeking an idle channel tothe same destination switching unit will be aware that the destinationswitching unit is receive busy for the current hub channel. Thus noconfusion arises out of substantially simultaneous requests for a hubchannel by different switching units to the same destination.

At the tick terminating one hub channel and starting the next, the bytein the THIS-SU register 532 is transferred into the hub-out dataregister 548 and the activity bit is transferred into the receiveactivity memory 543. The receive activity bit placed in the receiveactivity memory 543 is propagated on the activity line 541 during thenext frame of the same hub channel. The byte in the hub-out dataregister 548 is transferred to the hub-out data series register 549 andtransferred in series by the HIP to the memory 516.

If for a hub channel the origin switching unit is transmit busy or itfinds from the activity bit in the THIS-SU register 532 that itsintended destination is receive busy, the origin switching unit must trythe next hub channel as a possibility. If an origin switching unitexamines all of the hub channels and fails to find a channel in which itis transmit idle and the intended destination switching unit is receiveidle, then no suitable free hub channel exists at the moment on thatbus. With the hub switch configuration shown in FIGS. 15 and 16, theorigin switching unit may try the other hub switch element 517 and hubring bus 503. The origin switching unit may also try other destinationswitching units which can interconnect with the proper link group, orthe channel searching procedure may be repeated until a free hub channelis found. Repeating the procedure of searching for a free hub channelmay provide a channel with only slight delay, since typically a hubchannel is assigned and held for only a few hundred milliseconds; thatis, the length of a single burst. A free hub channel may becomeavailable when either the origin switching unit becomes transmit idle ina hub channel or the destination switching unit becomes receive idle ina hub channel.

Detailed Operation of the Hub Switch

The operation of a switching unit 501, particularly a hub switch element515 under control of the associated HIP and HOP, during the ringcirculation period of a hub channel on hub ring bus 502 will now beexplained in greater detail. The table of FIG. 20 provides a summary ofthe operations within a hub switch element during a hub channel undervarious sets of conditions.

The ring circulation period of one hub channel ends and that of the nextone starts at tick 0 (or tick N) of the central clock 505 of the hubswitch as illustrated in the diagram of FIG. 18. On this tick each byteis transferred into the THIS-SU register 532 of its destinationswitching unit from the preceding switching unit. As indicated in column1 of the table of FIG. 20 the byte is placed in the hub-out dataregister 548, and the activity bit (busy) is placed in the receiveactivity memory 543. The HIP transfers the byte to the memory 516 by wayof the hub-out data series register 549 to be processed by a LOP fortransmission on an outbound link channel.

If the byte placed in the hub-out data register 548 is a terminationcharacter which is to be used by the destination to determine that theburst has been terminated, the activity bit in the hub-out activityregister 548 will show a busy status. This information is processed bythe HIP to recognize the end of the burst. The hub channel status willbe changed to idle by the origin switching unit during a subsequentframe of the hub channel.

Also on the starting clock tick the bit in the receive activity memory543 indicating whether the switching unit is receive busy or receiveidle in the next hub channel on hub ring bus 502 just starting is passedthrough the multiplexer 533 to the activity line 541 to the successiveswitching unit. These actions are summarized in column 1 of the table ofFIG. 20.

On the first tick (0 or N) of a hub channel the destination counter 531is set to the switching unit's own address. If the switching unit haspreviously seized the hub channel just starting (during a previousframe), the transmit activity memory 559 contains a transmit busy bitfor the hub channel and the destination memory 540 contains the addressof the destination switching unit for the hub channel. The current hubchannel of the C hub channels is designated by a hub channel counter 550which is advanced every tick 0. A byte is loaded from the memory 516into the hub-in data register 535 by way of the hub-in data seriesregister 539 by the HOP. A bit indicating that a channel is needed isplaced in the need channel register 545 by the HOP. The address of thedestination switching unit is loaded into the destination register 536from the destination memory 540.

Upon each subsequent tick the destination counter 531 counts down byone, and a byte and an activity bit are loaded into the THIS-SU register532. If no match is produced between the contents of the destinationcounter 531 and the destination register 536 as indicated by the outputof the comparator 537, no action is taken by the switching unit withregard to the byte (if any) in the hub-in data register 535. On the nexttick of the clock the contents of the THIS-SU register 532 are passedthrough without change to the successive switching unit by themultiplexer 533. This situation is shown in column 2 of the table ofFIG. 20.

On a tick when the destination counter 531 is decremented to equal thecontents of the destination register 536, the output of the comparator537 indicates a match. It is assumed that the switching unit has seizedthe hub channel during a previous frame for transmission of the bytes ofthe burst to the destination address in the destination register 536. Inresponse to the indication of a match from the comparator 537 togetherwith a channel needed indication from the need channel register 545, achannel receive busy bit from the THIS-SU register 532, and a transmitbusy bit from the transmit activity memory 559, the channel acquisitionand data transfer section cause the multiplexer 533 to transfer thecontents of the hub-in data register 535 together with a busy activitybit to the successive switching unit. This situation is indicated incolumn 3 of the table of FIG. 20.

If the first byte of a burst is to be transferred from the memory 516 tothe hub ring, the switching unit must acquire a hub channel in which itis transmit idle and the destination switching unit, designated by theaddress information in the header of the burst, is receive idle. Asexplained previously on the first tick of the hub channel thedestination counter 531 is set to the address of the origin switchingunit. The HOP transfers the address information of the intendeddestination switching unit to the hub-in data series register 539. TheHOP also sets the need channel register 545 to indicate a channel isneeded. In response to the combination of the need channel indicationfrom the need channel register 545, a channel transmit idle indicationfrom the transmit activity memory 559, and an idle or inactive statusindication from a seize status register 551 indicating that no hubchannel search is underway, the address information in the hub-in dataseries register 559 is transferred to a new destination register 552,and the seize status register 551 is changed to indicate that the newdestination information has been received. Next, the first byte of theburst which is to be transmitted, which byte was transferred to thehub-in data series register 539 as the address information wastransferred out, is loaded into a first byte register 555. The seizestatus register 551 is changed to provide an indication that a channelsearch is underway for this hub channel. At the same time a try counter556 is loaded with a value C, specifically 32, the total number of hubchannels in a frame. Then the address information in the new destinationregister 552 is loaded into the destination register 536, and the bytein the first byte register 555 is loaded into the hub-in data register535.

On the tick of the clock at which the destination counter 531 isdecremented to equal the contents of the destination register 536, thecomparator 537 produces an indication of a match to the channelacquisition and data transfer section 538. The activity bit in theTHIS-SU register 532 indicates to the channel acquisition and datatransfer section 538 whether the destination switching unit is receiveidle or busy for this hub channel. If the activity bit in the THIS-SUregister 532 indicates idle status, the channel acquisition and datatransfer section 538 causes the multiplexer 533 to transfer the firstbyte of the burst in the hub-in data register 535 and a busy activitystatus bit to the successive switching unit. The channel seized register546 is set indicating that data has been inserted on the hub ring, andto inform the HOP that a hub channel has been seized and that subsequentbytes of the burst should be sent in subsequent frames of that hubchannel. In addition a transmit busy bit for this hub channel is storedin the transmit activity memory 559, and the address of the destinationswitching unit is transferred from the destination register 536 to thedestination memory 540 for use in subsequent frames. The seize statusregister 551 is restored to the inactive status. This situation issummarized in column 4 of the table of FIG. 20.

If on the tick of the ring circulation period when the comparatorindicates a match the activity bit in the THIS-SU register 532 indicatesbusy, this hub channel to the destination switching unit has alreadybeen acquired by another switching unit. Under these conditions assummarized in column 5 of the table of FIG. 20, the byte in the hub-indata register 535 is not transferred to the hub ring. Instead, thecontents of the THIS-SU register 532 are passed to the successiveswitching unit. The channel seized register 546 is not changedindicating to the HOP that the channel has not been seized. No change ismade in the transmit activity memory 559 or in the seize status register551, which still indicates that a search is underway.

On tick 0 (or tick N) at the start of each subsequent hub channel thepresence of the search underway indication from the seize statusregister 551 causes the try counter 556 to be decremented by one. On thenext hub channel that is not transmit active, (the transmit activity bitin the transmit activity memory 559 is idle), the contents of the firstbyte register 555 and the new destination register 512 (which remainuntil replaced) are placed in the hub-in data register 535 anddestination register 536, respectively. The hub switching element isthus primed to make another attempt to seize a hub channel when thecount in the destination counter 531 equals the contents of thedestination register 536. If the contents of the try counter 556 reach avalue of zero, it indicates that all C hub channels on the hub ring bus502 have been checked and none is currently available. When the trycounter 556 reaches zero, the seize status register 551 is reset to theinactive status. The HOP also counts the number of channels which haveoccurred since it delivered the first byte to the hub switching element.Thus, the HOP also knows when the search has been discontinued, since ithas not received a hub channel seized indication from the channel seizedregister 546 for C hub channel periods.

When all the bytes of a burst have been sent, the origin switching unittransmits a sequence of termination characters on the hub channel to thedestination switching unit to indicate the termination of the burst.Upon receiving this notification that the burst has been completed, theHIP of the destination switching unit manages the contents of its memory516 to reflect this situation. On the start of the hub channel duringthe next frame a receive idle activity bit in the receive activitymemory 543 is passed along the activity line 541 indicating that theswitching unit is now receive idle in that hub channel.

SWITCHING PROCESSOR

A burst switch is an intelligent switching network. When a burst isintroduced into the network through a port, the burst will be routed bythe network nodes to the burst's destination port as designated in theheader. The network directs the burst toward its destination portwithout external control intervention. This distributed networkswitching intelligence is provided essentially by two specially designedhigh-speed processors: a switching processor; and a queue sequencer.There are several variations of the switching processor which differonly in firmware. A burst switch generally has other high-level controlprocessors which, for example, determine what the content of a burstheader should be. Here, the focus is directed only to the intelligenceassociated with the in-line switching functions; the processors whichperform these functions will be referred to as switching processorsexcept for the queue sequencer which will be referred to by name. Eachswitching processor works in conjunction with the queue sequencer.

The operations of the various switching processors in a link switch andhub switch have already been described. In a link switch, the switchingprocessors mediate the flow of bytes of a burst between communicationslinks and central memory and between ports and central memory. In thehub switch, the switching processors mediate the movement of bytesbetween a communications link and central memory and between acommunications hub and central memory.

A general purpose processor can be programmed to perform the functionsof a switching processor. An experimental burst switch was constructedat GTE Laboratories with Rockwell 6502 Microprocessors as its switchingprocessors. However, this experimental model supported only fourchannels on its communications links. It is highly desirable that aburst switch support twenty-four, thirty-two, or more channels on itscommunications links.

The speed requirements imposed on the switching processors are quitestringent. As explained above, a burst switch has in-line switchingcapability in the sense that substantially all of the processingrequired for an incoming or outgoing byte must be completed within asingle channel time, so that the switch can stay current in real time. AT1-carrier with twenty-four channels has a channel time of 5.2microseconds. In a carrier with thirty-two channels, as is the Europeanstandard, the channel time is 3.9 microseconds. It has been determinedthat a switching processor must be capable of performing approximatelyfifty operations per channel time in order to stay current. In view ofthis stringent speed requirement, specially designed switchingprocessors are required to implement a full complement of channels.There is no commercially available microprocessor which can performapproximately fifty operations per 5.2 or 3.9 microsecond period.

As has been explained previously, a typical link switch has six or eightswitching processors competing for access to central memory. The controllogic requires that only one processor may obtain access at any time.Accordingly, it is necessary that memory access be arbitrated. As aresult of memory arbitration, a switching processor may have to wait,while another processor completes its use of memory, thus reducing theavailable in-line processing time. Since this waiting time may besubstantial under certain conditions, there is a need for a parallelprocessing capability.

The central memory of a link switch contains the dynamic buffers andqueue headers used in processing bursts. Memory accesses by the variousswitching processors are primarily of two types: to read or write acharacter (or byte) of a burst; and to administer queues and buffers.

Memory contention can be reduced by dividing central memory into twoparts: a character memory, which contains only characters; and a queuememory, which contains queue and buffer administrative information (notthe characters of the various queues). Only one character may be read orwritten from character memory during a single memory access. During asingle operation on queue memory, a number of uninterrupted reads and/orwrites is permitted (which may be required to link multiple buffers of aqueue).

The administration of queues and buffers is a specialized functioncommon to all switching processors. A specialized processor, called aqueue sequencer, has been added to administer all information in thequeue memory on behalf of all switching processors. When a switchingprocessor requires a queuing action, it delegates such action to thequeue sequencer; thereafter, the switching processor is free to performfurther processing without awaiting access to queue memory. The actionsof the queue sequencer are carried out in parallel with those of theswitching processors. Thus, the addition of the queue sequencer to thelink switch provides two speed advantages: first, queue administrationis conducted in parallel thereby relieving the switching processors of asubstantial in-line responsibility; second, the waiting time for theswitching processors to access queue memory has been eliminated. Inaddition to the speed advantages, the queue sequencer providessubdivisions of actions on the queue memory. This means that a series ofuninterrupted reads and/or writes is permitted for each operation onqueue memory. Thus, additional accesses and processing, otherwise neededto insure no interference between switching processors, are notrequired.

FIG. 21 contains a block diagram of a typical link switch 600 coupledbetween time-division multiplexed communications links 630 and 632showing the queue sequencer and various embodiments, or firmwarevariants, of the switching processor. Each input or output switchingprocessor, LIP 610, LIP 612, PIP 614, LOP 604, LOP 620, or POP 618, hasan interface with queue sequencer bus (QS Bus) 634, with charactermemory bus (CM Bus) 636, and with a communications link or port bus.Queue sequencer (QS) 608 mediates QS bus 634 and queue memory (QM) 602.Queue memory 602 in FIG. 21 is shown conceptually as a componentseparate from queue sequencer 608. Character memory (CM) 606 is coupledwith CM bus 636. Each port interface (PI) circuit 622 (twenty-four portsare indicated in the drawing) is coupled with PI input bus 640 and PIoutput bus 638. PI-In bus 640 and PI-Out bus 638 are coupledrespectively with PIP 614 and POP 618. QS Bus 634 and CM Bus 636 arearbitrated buses. Each block of FIG. 21 having a triangle 642 in theupper right corner is a firmware variation of the basic switchingprocessor. The basic switching processor is also employed incounterparts of the hub switch.

Arbitration is required on both CM Bus 636 and QS Bus 634 in order toinsure that no collisions or data corruption occur on these buses andthat all requests for actions on CM 606 and QM 602 are serviced fairly.Contention on each of these buses may be resolved using priorityarbitration logic, appropriately modified, as shown in FIG. 3A. In FIG.21, CM 606 and QM 602 each include timing control for generatinginternal read and write pulses, and random access memory. All transfersacross CM Bus 636 and QS Bus 634 take one machine cycle. Arbitration forone cycle may be performed concurrently with bus transfers, so that onetransfer may be performed every cycle.

FIG. 22 shows a block diagram of the architecture of the basic switchingprocessor 700. Control 710 executes instructions from program memory702. In a preferred embodiment, program memory 702 is implemented as aprogrammable read only memory, designated PROM in the drawing. There areregisters 704, an arithmetic/logic unit (ALU) 706, and data memory,i.e., random access memory 708. There is an interface with the queuesequencer bus (QS-IF) 714; an interface with the character memory bus(CM-IF) 718; and an external interface (EX-IF) 720 which provides meansfor coupling with a communications link, port, or hub. Dual-port RAM(DP-RAM) 716 comprises means for providing the current buffer address incharacter memory (as a function of present channel number). Finite statemachine (FSM) 722 comprises means for determining the status of eachchannel with respect to incoming bursts. Typical channel states of FSM722 are: channel idle, awaiting a burst; particular header bytereceived; informational byte received; DLE (data link escape) bytereceived; and FLAG (end of burst) character received.

Each component of switching processor 700 is coupled with data/addressbus 712 except for PROM 702 and DP-RAM 716. PROM 702 is coupled withcontrol 710 via instruction bus 724 and microcode address bus 726.DP-RAM 716 is coupled between QS-IF 714 and CM-IF 718 by means of buses728 and 730, respectively. FSM 722 is coupled with EX-IF 720 via bus732. FSM 722 also is coupled with control 710 via jump address bus 734.As shown in the drawing, there are control lines from control 710 toeach component. Control 710 includes channel counter means, e.g., abroadcast channel count or a count derived from a broadcast channelclock. Control line 736 provides an input to control 710 from anexternal channel clock source.

The instructions executed by switching processor 700 are read from PROM702 via instruction bus 724. Control 710 provides the address of thenext instruction to be executed via address bus 726. For each embodimentof the switching processor, the microcoded programs will not change.Therefore, the program memory is a read only memory.

PROM 702 comprises 256 words, each word having a length of 64 bits. Thisword length is greater than that typically found in the art. Theextended word length provides a speed advantage in several respects.More than one operation may be included in a single instruction word,e.g., a register transfer and an ALU operation, so that severaloperations can be performed within the time allotted for a singleinstruction. Various bit positions in the instruction word have beenassigned to certain registers, operations, etc., so that decoding ofprogram instructions has been minimized. For example, a register may beaddressed by the presence of a bit in the assigned bit position of theinstruction word. Operations on several registers may occur within thesame instruction cycle. Instructions are executed in a pipelined manner.Instruction fetch overlaps instruction execution; during the executioncycle of a particular instruction, the next sequential instruction isfetched. Thus, the switching processor executes one instruction percycle. The next sequential instruction is the next instruction to beexecuted unless the current instruction is a jump instruction. Jumpinstructions introduce a non-sequential address for the next instructionand therefore require two cycles to take effect.

RAM 708 comprises 1024 bytes. It serves as the local data memory forswitching processor 700. It contains various status variables andparameters for each of the switching processor's channels, e.g., thereis an indicator to denote whether or not a burst has been routed towardits destination. The data memory address is a concatenation of thechannel counter (5 bits) and the offset (5 bits) within control 710available from the instruction.

ALU 706 performs standard arithmetic and logic operations.

Dual-port RAM 716 is the switching processor's storage for active bufferaddresses. The switching processor sends the buffer address to the queuesequencer or character memory by addressing DP-RAM 716 with the channelcounter; this reads out the active buffer for that channel, which isautomatically sent to the queue sequencer or character memory. When thequeue sequencer updates the active buffer for a given channel, it doesso by addressing DP-RAM 716 with the channel number and writing the newbuffer address. DP-RAM 716 may be implemented using a commerciallyavailable dual-port RAM or by using a single-port RAM with multiplexingcircuitry on the address and data buses together with contention controllogic.

Registers 704 comprise eighteen registers each accessing the eight-bitinternal data bus 712. Most registers contain eight bits. The switchingprocessor registers are listed in Table 1 as follows.

                  TABLE 1                                                         ______________________________________                                        Switching Processor Registers and Latches                                                                         Write Read                                No.  Abbrev.  Name             Bits DB*   DB*                                 ______________________________________                                        1    ALUO     ALU Output Register                                                                            8    X                                         2    AREG     ALU Input Register                                                                             8          X                                   3    BTYP     Burst Type       3          X                                   4    BUFF     Buffer Register  8    X                                         5    CHNS     Channel Status Register                                                                        4          X                                        In                                                                       6    CHNS     Channel Status Register                                                                        4          X                                        IN                                                                       7    CHRS     Character Status Out                                                                           4    X                                              OUT      Register                                                        8    INDX     Index Register   8          X                                   9    INPUT    Input from external                                                                            8    X                                                       interface                                                       10   INPUT    Input Strobe Latch          X                                        STR                                                                      11   LMEM     Local Memory In  8          X                                        IN                                                                       12   LMEM     Local Memory Out 8    X                                              OUT                                                                      13   OUTP     Output to external                                                                             8          X                                                 interface                                                       14   OWNS     Own Switch ID    5    X                                         15   QUEU     Queue Register   5          X                                   16   QS REQ   Queue Sequence Request                                                                         8          X                                   17   SEQD     Data Out Field from                                                                            8    X                                                       Microword                                                       18            Channel Counter  5    X                                         ______________________________________                                         *DB = Data Bus 712                                                       

QS-IF 714 is the switching processor's interface to the queue sequencerbus. When switching processor 700 requires a queue sequencer function,control 710 issues a command or request via QS-IF 714. Control 710merely issues the command or request. QS-IF 714 has means for gainingaccess to the arbitrated queue sequencer bus and transferring therequest to the queue sequencer. The queue sequencer executes the requestindependently of the switching processor. The queue sequencer will, whenappropriate, return a single piece of information to switching processor700, namely the address or number of a new buffer in character memory tobe used with a particular channel. This buffer address passes from QS-IF714 directly to DP-RAM 716 via bus 728 and is stored there at thelocation addressed by the channel number.

The input switching processors store bytes or characters into charactermemory; the output switching processors read bytes or characters fromcharacter memory. For each access, a buffer address for the currentchannel is required. The buffer address comprises the buffer number,supplied by DP-RAM 716, concatenated with the position in buffer orindex, stored in RAM 708. CM-IF 718 has means for implementing theaccess to or from character memory independently after the bufferaddress has been supplied.

Each switching processor serves as an intermediary between charactermemory and a communications link, port, or hub. EX-IF 720, referred toas the external interface, provides interface means for a link, port, orhub, depending on the particular embodiment of the switching processor.In a switching processor on the input side of central memory, EX-IF 720has means for providing the received byte to FSM 722 via bus 732.

The purpose of FSM 722 is to assist control 710 by performing logicassociated with incoming bursts in parallel with control 710. Once thestatus of an incoming byte has been determined by FSM 722, a jumpaddress is placed on bus 734. This jump address is the location of themicrocoded subroutine in PROM 702 which is appropriate for processingthe incoming byte. This type of jump is typically performed once perchannel time. The appropriate jump address is determined by FSM 722essentially by two criteria or states: character state, and channelstate. As will be seen, the status of an incoming byte in general isdependent on the status of the preceding byte of the same burst orchannel. The status relationships are best illustrated in statediagrams.

FIG. 23 shows character state diagram 750. There are three characterstates: Clear, in block 752; DLE-Found, in block 754; and FLAG-Found, inblock 756. Each state is determined by the incoming byte and the statusof the preceding byte of the same burst or channel. The process isinitialized in the Clear state. If a byte other than a DLE or FLAGcharacter is received, the status remains Clear. If the received byte isa DLE, the status changes to DLE-Found. If the received byte is a FLAG,the status changes from Clear to FLAG-Found. The Clear state indicatesthat either type of burst characters may be received, i.e., either acontrol character or an informational character. The two controlcharacters are DLE and FLAG.

When status is DLE-Found, the status will return to Clear upon receptionof any byte. Any character following a DLE will be interpreted as aninformational character rather than a control character.

When status is FLAG-Found, reception of any character other than anothercontrol character will return the status to Clear. If another FLAGcharacter is received, the status remains in FLAG-Found. If a DLEcharacter is received, the status changes to DLE-Found. Since a DLEcharacter indicates that the following character is a data character,there is no possibility of passing from DLE-Found directly toFLAG-Found. FLAG indicates end-of-burst or an idle channel. Thetransition from FLAG-Found to Clear occurs upon receipt of the firstnon-control character, i.e., not FLAG and not DLE.

FIG. 24 shows channel state diagram 800. This diagram shows the flow ofcontrol between channel states while processing an incoming burst. Theinitial state is that of awaiting reception of the first byte of a burstheader in block 802. If either control character, (DLE or FLAG), isreceived, the status does not change. In a preferred embodiment, FLAGcharacters are transmitted in an idle channel. When the first byte of aburst has been received, the status changes to that of awaiting thesecond header character in block 804. Likewise, the status changes tothat of awaiting the third and fourth header bytes in blocks 806 and 808respectively upon receipt of a non-control character in either case. Inblocks 804, 806, and 808, the status remains unchanged upon receipt of aDLE.

If an FLAG is received during the processing of the header, there is anerror in the header (which contains the destination of the burst). Withthe destination in doubt, the transmission of the burst is aborted asshown in block 814. For the identical reason, the transmission of aburst is aborted if the burst-header check character (BCC) does notvalidate after receipt of the fourth header byte as indicated by arrow818. (The BCC has been referred to above as the header check sequence.)When a burst transmission has been aborted, the bytes of the burst areprocessed without retransmission in block 814 until FLAG is receivedwhereupon the status changes to that of awaiting receipt of thetermination sequence in block 812. If the system termination sequence isa single FLAG, control passes directly through block 812 and returns tothe idle state in block 802. If the termination sequence is more thanone character, e.g., at least three FLAGS in five characters, controlremains in block 812 until the successful reception of the terminationsequence.

After reception of the fourth header byte in block 808 and successfulvalidation of the burst check character, control passes to block 810 inwhich the informational portion of the burst is processed. Uponreception of FLAG, the status changes to that of awaiting thetermination sequence in block 812.

A control burst is one which is sent between switches, rather thanbetween end users, for purposes of internal administration. An exampleof a control burst is a change in the routing table of a switch. Acontrol burst may be detected in block 808. If a control burst isreceived, status passes to block 816 in which the control burst isprocessed. Upon receipt of a FLAG, control passes from block 816 toblock 812 where the termination sequence, if any, is awaited.

Finite state machine 722 has three character states, shown in the threeblocks of FIG. 23, and eight channel states, shown in the eight blocksof FIG. 24. Each block of both figures corresponds to a unique state ofFSM 722. Referring to FIG. 22, the logic for processing incoming burstsis inextricably linked between control 710 and FSM 722. Control 710initializes FSM 722. Upon receipt of an incoming byte, FSM 722determines its proper state and places the jump address corresponding tothis state on bus 734. As used herein, the term "bus" includes asingle-entry single-exit data path as in bus 734 and elsewhere. Control710 jumps to the address on bus 734 which is the location of theappropriate microcoded subroutine for processing the incoming byte.After processing the byte, control 710 provides feedback to FSM 722 viadata/address bus 712. Employing the feedback provided by control 710,FSM 722 determines the proper state for reception of next byte in thesame channel of the next frame. Thus, each component providesinformation to the other which is vital to the proper functioning of theswitching processor.

The basic instruction set of the switching processor is listed in Table2. As shown in Table 2, there are four groups of instructions: moves;arithmetic logic unit instructions; jumps; and miscellaneousinstructions.

                  TABLE 2                                                         ______________________________________                                        Switching Processor Instruction Set                                           ______________________________________                                        mov  move from register to register(s)                                             move from register to memory                                                  move from memory to register(s)                                          alu  $inc    increment AREG, to ALUO                                          alu  $incq   increment AREG, to ALUO and QREG                                 alu  $deq    decrement AREG, to ALUO and QREG                                 alu  $atoq   transfer AREG to QREG                                            alu  $exor   Exclusive OR of AREG & QREG, to ALUO                             alu  $and    Logical AND of AREG & QREG, to ALUO                              alu  $andq   Logical AND of AREG & QREG, to ALUO and QREG                            where AREG = ALU input register                                                     ALUO = ALU output register                                                    QREG = ALU internal register                                     Normal Jumps (jump address from the microcoded PROM):                         jnu  jump unconditionally                                                     jne  jump if equal                                                            jnn  jump if not equal                                                        jnw  jump if wait flag active                                                 jnb  jump if no buffer                                                        jns  jump if not start of channel                                             jnq  jump if queue sequencer acknowledges                                     jnc  jump if character memory acknowledges                                    jnl  jump if link processor (LIP or LOP)                                      FSM Jumps (jump address from the Finite State Machine):                       jfu  jump unconditionally                                                     jfe  jump if equal                                                            jfn  jump if not equal                                                        jfw  jump if wait flag active                                                 jfb  jump if no buffer                                                        jfs  jump if not start of channel                                             jfq  jump if queue sequencer acknowledges                                     ifc  jump if character memory acknowledges                                    jfl  jump if link processor (LIP or LOP)                                      nop  no operation                                                             req  request work of queue sequencer                                          rst  reset channel start latch                                                ______________________________________                                    

The general operation of a switching processor is as follows.

1. The channel count is advanced, and the current channel's parametersare fetched from local memory.

2. The next byte is inputted, from the external interface in inputprocessors; or from character memory in output processors.

3. Processing is done based upon the channel state and the receivedbyte.

4. The next character is outputted, to character memory by inputprocessors; or to the external interface for output processors.

5. The appropriate request is issued to the queue sequencer via thequeue sequencer interface. Each switching processor issues a queuesequencer request each channel time.

QUEUE SEQUENCER

The queue sequencer is a memory management processor in a link switch orin the link interface of a hub switch. A link switch includes input andoutput processors which operate independently, communicating with eachother via a common character memory. An input processor receivesinformation from an incoming communications link or port and stores itin a linked buffer in character memory.

An output processor then removes the information from character memory,and places it on an outgoing link, port, or hub. The queue sequencerprovides management of buffers and connects input processors to outputprocessors by assigning the appropriate buffers to each.

The queue sequencer uses a linked-list data structure to manage buffers.Buffers are placed in a queue corresponding to the destination and bursttype of the burst they contain. All unused buffers are placed in aseparate queue, called the free queue. When a burst is stored for aperiod longer than a frame time times the length of a buffer, the bufferis chained to another buffer in such a way as to provide anunlimited-length elastic storage.

Upon initiation of a burst, an input processor issues an "enque" requestto the queue sequencer which places an entry into the output queuecorresponding to the burst. When approaching an idle output channel, anoutput processor issues a "deque" request to the queue sequencer,whereupon the queue sequencer provides the address of the highestpriority burst awaiting an output channel. As used herein, the term"enque" is an abbreviation for "enqueue," which means adding an entry toa queue; the term "deque" is short for "dequeue," which means deletingan entry from a queue.

Before the beginning of a burst, an input processor issues a "getbuf"request to the queue sequencer, and the queue sequencer responds withthe address of the next available buffer from the free-queue list. Aftersending the last character of a buffer, an output processor issues a"putbuf" request which directs the queue sequencer to place the bufferon the free queue list.

The queue sequencer administers the chaining of buffers for bursts whichhave been queued for a long period of time. Upon storing a datacharacter in the character memory, the input switching processor issuesan "inccon" request to the queue sequencer. The queue sequencer thenreturns a new buffer to the switching processor if buffers have beenchained. Similarly, an output processor requests a "deccon" whileprocessing data characters of a burst. If buffers have been chained, thenew buffer address is provided to the output processor by the queuesequencer.

The queue sequencer performs queue administration independently of andsubstantially in parallel with the various switching processors. Atinitialization, the queue sequencer places all of the (empty) buffers incharacter memory on the free queue list. During operation, the queuesequencer allocates and surrenders buffers from and to the free queuelist for the various switching processors. When there are no new burstsawaiting output, the output queues are empty.

FIG. 25 shows block diagram 850 describing the architecture of the queuesequencer. Control 860, registers 854, ALU 856, and RAM 858 are eachcoupled with internal data/address bus 862. PROM 852 is coupled withcontrol 860 via instruction bus 876 and address bus 874. These blocksperform essentially the same functions as described above with regard tothe switching processor. RAM 858 in FIG. 25 is shown as an internal partof the queue sequencer. In FIG. 21, QM 602 is shown conceptually as aseparate memory component apart from the queue sequencer. Thedescription in FIG. 25 is a preferred embodiment.

The queue sequencer interfaces with the switching processor via thequeue sequencer bus, denoted as QS-BUS in the drawing. This interface isdivided into two portions, input and output. Output interface, OUT-IF864, is coupled with data/address bus 862. Input interface, IN-IF 866,is coupled with first-in first-out buffer, FIFO 868, which acts as ininput buffer between the switching processors and queue sequencer.Various switching processors independently issue requests to the queuesequencer.

Whenever a switching processor gains access to the queue sequencer bus,it stores a request on "top" of FIFO 868. Whenever the queue sequencercompletes processing a request, it obtains its next control from the"bottom" of FIFO 868, if there is one. If there are no requests in FIFO868, the queue sequencer awaits receipt of a request in an idle loop.

FIFO 868 stores requests from the various switching processors infirst-come first-serve order within priority groups. These FIFOs may beimplemented using commercially available FIFO integrated chips andcontrol logic. The output of each FIFO is the next pending request. Thisrequest is mapped through a look-up table into an address in PROM 852,which is used by control 860 as a jump address to the microcode routinecorresponding to that request. Queue sequencer 850 contains two sets ofFIFOs: one for normal priority requests, and another for high priorityrequests. If there is any request in the high priority FIFO, its outputis taken as the next pending request. Otherwise, the output of thenormal priority FIFO is used.

Enque/Deque 870 contains logic which is used to update the index ofpending work for each destination of an output queue. When a burst isenqued to a destination which has no other bursts of that burst typepending, a bit corresponding to the burst type must be set in the indexfor that destination. This is done by decoding the burst-type field,ANDing the result with the index, and storing the final result as theindex. When an output processor requests a deque, Enque/Deque 870performs two tasks. First, the highest priority burst type pending mustbe extracted from the index register; this may be done using a priorityencoder and a decoder. Second, the bit corresponding to the burst typein the index register must be reset if the burst being dequed is thelast one pending of that burst type. This may be done by ORing the indexwith the complement of the highest priority burst type pending. Theoperations performed by the Enque/Deque logic could also be performed bycontrol 860 in several instructions, or could be done using a custom ALUwith the described operations incorporated into the logic.

In the queue sequencer, PROM 852 has 256 words, each word having alength of 64 bits. The speed and decoding advantages realized from thelength of the instruction word are the same as described for theprogrammable read only memory of the switching processor. RAM 858includes 2,048 bytes of data memory. Two types of information are storedin RAM 858, as follows: administrative information on each buffer incharacter memory; and administrative information on each queue in theswitching processor memories.

The arithmetic and logical operations of ALU 856 include means forincrementing and decrementing. Registers 854 access eight-bit internaldata and address buses 862. The queue sequencer registers are specifiedin Table 3 as follows.

                  TABLE 3                                                         ______________________________________                                        Queue Sequencer Registers                                                                                     To   From  To                                 No.  Abbrev.  Name         Bits DB*  DB*   AB**                               ______________________________________                                        1    AND      Removes bit from                                                                           8    X                                                           IREG                                                            2    BTYP     Burst Type   3               X                                  3    BUFF     Buffer Register                                                                            8    X          X                                  4    BUFO     Buffer Out   8         X                                                      Register                                                        5    CNTR     Counter      8    X                                             6    DREG     Data Register                                                                              8    X    X     X                                  7    IREG     Index Register                                                                             8         X     X                                  8    MMEM     Main (Local) 8    X    X                                                      Memory                                                          9    QUEUE    Queue Register                                                                             8               X                                  10   SEQU     Data Out Field in                                                                          8    X          X                                                Microword                                                       11   TEMP     Temporary    8    X    X     X                                                Register                                                        12   XNOR     Adds bit to IREG                                                                           8    X                                             ______________________________________                                         *DB = Data Bus                                                                **AB = Address Bus                                                       

In order to reduce the load on the central control of the switchingprocessors and queue sequencer, the various interfaces of a link switchhave been designed to perform bus transfers independently. Thistechnique is known in the art. FIG. 25A is a block diagram 900 of aninterface circuit employing handshaking logic which with appropriateadaptation may be used as any of the interfaces in the switchingprocessor and queue sequencer.

FIG. 25A shows an interface between two processors A and B. Tocommunicate with processor B, processor A places data to be sent on itsinternal bus 902 and activates send line 904. The timing of the transferacross external bus 906 is controlled by bus arbitration logic 908. Toreceive the data, processor B activates receive line 910 and reads thedata from its internal bus 912.

This type of interface may be operated synchronously or asynchronously.It may be used to communicate between logic on different processor, aswell as between buses, in which case latch A, latch B, or both might bereplaced by a buffer. In FIG. 22, QS-IF 714 and CM-IF 718 are of thelatter type. In FIG. 25, Out-IF 864 and In-IF 866 are also of the lattertype.

The basic instruction set of the queue sequencer is listed in Table 4.

                  TABLE 4                                                         ______________________________________                                        Queue Sequencer Instruction Set                                               ______________________________________                                        mov - Move from:                                                                          register to register(s)                                                       register to memory                                                            memory to register(s)                                             Normal Jumps (jump address from microcode PROM):                              jnu  Jump unconditionally                                                     jne  Jump if equal                                                            jnn  Jump if not equal                                                        jng  Jump if greater than                                                     jnl  Jump if less than                                                        FIFO Jumps (jump address from control in FIFO):                               jfu  Jump unconditionally                                                     jfe  Jump if equal                                                            jfn  Jump if not equal                                                        jfg  Jump if greater than                                                     jfl  Jump if less than                                                        nop  No operation                                                             nst  Normal Strobe; get next normal control from FIFO                         pst  Priority strobe; get next priority control from                               FIFO                                                                     inc  Increment buffer count parameter                                         dec  Decrement buffer count parameter                                         ein  Enable burst addition to index register                                  din  Enable burst removal from index register                                 ______________________________________                                    

The operation of the queue sequencer is controlled by commands orrequests placed in FIFO 868 by the various switching processors. Eachrequest corresponds to a microcoded subroutine stored in PROM 852.

Upon startup, the queue sequencer initializes RAM 858. This taskconsists of setting all queues to idle, setting all buffers to idle, andplacing all buffers on the free queue list.

The queue sequencer then enters its idle loop. In the idle loop, thequeue sequencer queries FIFO 868 to determine if a request has arrived.When a request is present, a FIFO jump instruction is executed. Thistransfers control to the beginning of the microcoded routine whichimplements the particular request.

At the conclusion of a routine, FIFO 868 again is queried for the nextrequest, if any. If there is another request, the proper FIFO jump isexecuted to implement the request. If there is no request the queuesequencer idle loop is resumed.

As has been described, the switching processor and queue sequencer workcooperatively to provide the speed advantage necessary to enable thelink switch (or the link interface of the hub switch) to perform in-lineintegrated switching. The speed advantage is realized by judiciousdesigns of architectures. The central memory has been divided into acharacter portion and an administrative portion thereby reducing memorycontention. The queue sequencer has been designed to operate in parallelwith the various switching processors and to perform specialized tasksrelating to the administrative portions of memory.

The wide instruction format of the switching processor and the queuesequencer permits direct activation of processor signals withoutdecoding delays. The wide instruction format permits more than oneprocessor operation to be performed concurrently. A shorter instructionword would incur additional decoding delay, and it would support onlyone operation per instruction. The pipelined mode of operation withinstruction fetch overlapping execution of the previous instructionincreases the speed of operation over a non-pipelined mode of operation.

The specialized interfaces of the switching processor and queuesequencer will continue operating to conclusion, once started. Thus,neither processor is delayed as a result of input/output tasks. Specialhardware is provided to speed actions which would require more time ifperformed in software. Examples of this specialized logic are the finitestate machine, the various interfaces, and the enque/deque logic.

The architectures of the switching processor and queue sequencer haveincorporated these features to optimize processing efficiency, so thatthe stringent time constraints of integrated in-line switching can beachieved.

Table 5 contains estimates of the speed-up factors of the switchingprocessor resulting from the special architectural features of thisprocessor. Table 6 contains similar speed-up estimates resulting fromthe specialized architecture of the queue sequencer. Table 7 containsspeed-up estimates of the architecture of the link switch or linkinterface of a hub switch. The entries in these tables are roughestimates made in comparison with similar architecture employing ahypothetical typical microprocessor of the existing art having a 6 MHzclock and without specialized support hardware. The factors in thesetables are neither independent nor mutually exclusive. Therefore, anoverall speed-up factor for the in-line switching functions cannot beobtained by computing the product of various factors. These tables areoffered as aids to understanding the architectures of the respectiveprocessors. Experience has shown that the overall speed enhancementobtained in the burst-switching embodiment is approximately twentytimes. This substantial speed-up factor enables execution of the variousin-line switching functions above described.

                  TABLE 5                                                         ______________________________________                                        Switching Processor                                                           Estimates of Speed-up Factors                                                 Feature      Factor  Mechanism                                                ______________________________________                                        Reduced Instruction                                                                        5       Faster cycle time through:                               Set                                                                           No External Memory Access                                                     Simple Instructions                                                           Less Instruction Decoding                                                                          Fewer Cycles per Instruction                             Parallelism in                                                                             2       Fewer Instructions Required                              Instructions         (by Approx. 50%)                                         Finite State 1.5     Fewer Instructions Required                              Machine for          (by Approx. 33%)                                         Character and                                                                 Channel States                                                                "Independent"                                                                              1.3     No Need to Await Bus Grant                               Interfaces           in Software                                              Dual-port RAM                                                                              1.3     No Need to Await Interrupt                                                    from Queue Sequencer                                     ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Queue Sequencer                                                               Estimates of Speed-up Factors                                                 Feature      Factor  Mechanism                                                ______________________________________                                        Reduced Instruction                                                                        5       Faster Cycle Time through:                               Set                                                                           No External Memory Access                                                     Simple Instructions                                                           Less Instruction Decoding                                                                          Fewer Cycles per Instruction                             Parallelism in                                                                             2       Fewer Instructions Required                              Instructions         (by Approx. 50%)                                         "Independent"                                                                              1.3     No Need to Await Bus Grant                               Interfaces           in Software                                              Enque/Deque  1.5     In Critical Path: One cycle                              Hardware             in lieu of 9 Instructions                                                     for Deque and 6 Instructions                                                  for Enque                                                ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Link Switch                                                                   Estimates of Speed-up Factors                                                 Feature     Factor    Mechanism                                               ______________________________________                                        Queue Sequencer                                                                           2         No Need for Switching                                                         Processors to Perform                                                         Serialization and Memory                                                      Management Functions                                    FIFOs on    1.5       No Need for Switching                                   Queue Sequencer       Processors to Await Queue                                                     Sequencer Readiness;                                                          Queue Sequencer may                                                           Balance Workload                                        Separate Queue                                                                            1.3       Parallel Access to Memories                             Sequencer and         Less contention on Each Bus                             Character                                                                     Memory Buses                                                                  ______________________________________                                    

The switching processor and queue sequencer are special purposesequencers. Both have microprograms which control a number ofspecialized registers, RAMs, and other hardware structures. Themicroprograms also contain flow-of-control information to direct jumpsand branches. The microprogram format is specific to the hardware beingcontrolled; there is a bit or a group of bits to control each hardwarefunction.

The microcode format for the queue sequencer is shown in FIG. 27. Themicrocode is 64 bits wide, and is organized as eight 8-bit bytes. Thefirst byte is the Upper Address byte, which is used to address the queuesequencer's local memory through the address bus. This bus can also becontrolled through a register, in which case the Upper Address Out byteis all ones. The second byte of the Microcode contains a Lower AddressParameter (LADDOUT) and a select code. The Lower Address parameter isused to address the local memory. The Select code determine's where thenext microcode address will come from, essentially controlling jumps andbranches.

The third byte of the microcode contains the NEXTADD parameter. This isused only when jumping, and contains the address of the instruction tobe jumped to. The fourth byte contains immediate data to be placed ontothe data bus. This allows constants to be introduced into any of theregisters from the microcode. The fifth byte of the microcode contains aNAEN bit and five write enable bits. The NAEN bit controls casebranches: when the bit is active, the next microcode address is takenfrom a mapping PROM which contains the next `routine` to be executed.This is analogous to fetching and decoding an instruction in amicroprogrammable computer. The write enable bits control the writing ofinformation into registers. When any of these bits are active, whateveris on the data bus is written into the designated registers. Any numberof registers may be written into simultaneously. There are five writeenables WENs: Index Register, IREG; Buffer Out Register, BUFO; Main(Local) Memory, MMEM; Data Register, DREG; and Temporary Register, TEMP.

The sixth byte of the microprogram word contains Data Read Bits. Undercontrol of these bits, the information in a register is placed onto thedata bus. Only one register may be read at a time. There are eight databits: the contents of the AND circuitry (AND removes a bit from theindex register); the XNOR circuitry (which adds a bit to the indexregister); the Counter, CNTR; the buffer register, BUFF; the DATAOUTfield in the microprogram word, SEQU; the Memory output, MMEM; the dataregister, DREG; and the temporary register, TEMP.

The seventh byte of the microprogram word contains Upper Address andMiddle Address bits. Under the control of these bits, information from aregister is placed on the Address Bus, UAENs, or part of the addressbus, MAEN. Only one UAEN or MAEN may be active at any given time. Thethree middle address registers are: burst type register, BTYP; indexregister, IREG; and data from the sequencer, SEQU. These place data onthe three least significant bits of the address bus. The five upperaddress enables place data on the entire address bus. These are: bufferregister, BUFF; sequencer data, SEQU; queue register, QUEUE; dataregister, DREG; and temporary register, TEMP.

The eighth byte of the microcode word contains miscellaneous controlbits. DQR1 and DQR0 signal the completion of deque requests to LOP1 andLOP0 respectively. B/Q controls the most significant local memoryaddress bit, thus addressing either buffer parameters or queueparameters. B/T controls the selection of either the index register orthe burst type register for the manipulation of the queue index (whichkeeps track of pending work by priority). D/U determines whether thecounter counts up or down, for manipulation of the buffer count for LIPsand LOPs respectively. CNTEN controls the action of the counter. PSTRand NSTR strobe the output register of the high priority and normalpriority FIFOs respectively.

FIG. 26 shows the microcode format for the switching processor. Theformat is similar to that of the queue sequencer, but differs in thefunction of many of the bits. The first byte contains the Address Outdata, which is only five bits for the switching processor. The mostsignificant bit of this byte controls the most significant address bitof the local memory, thus addressing either the channel parameters orthe switching processor's routing table. The second byte of themicrocode contains the next address field, and the third byte is thedata field. These are analogous to the same fields in the queuesequencer microcode.

The switching processor microcode has ten write enable bits: eight inbyte 4, and two in byte 7. They allow data to be written from the databus into ten different places: the local memory, LMEM IN; the burst typeregister, BTYP; the output register, OUTP; the Arithmetic Logic Unit'sinput register, AREG; the character status register, CHRS IN; thechannel status register CHNS IN; the index register, INDX; the queueregister, QUEU; the queue sequencer request latch, QS REQ; and the inputstrobe latch, INPUT STR.

The fifth byte of the microword contains the data enable controls. Theseare seven bits which allow data to be transferred from a register to thedata bus. The seven registers are: the data from the microword DATAOUTfield, SEQD; the switching processor's local address switch, OWNS; thecharacter status out register, CHRS OUT; the ALU output register, ALUO;the input register, IPUT; the buffer register, BUFF; and the localmemory, LMEM OUT.

The sixth byte of the microword controls the switching processor'sArithmetic Logic Unit. This involves a 6-bit coded control word whichdirect the ALU to add, subtract, shift, XOR, or perform anotherfunction. The seventh byte of the microword contains two write enables,a reset of the channel start latch, REST STRT; and a deque request latchcontrol, DEQ. The eighth byte contains bits to control the sequence ofinstructions. These are: a bit which brings the next address from theNEXTADD field in the microword, EPA SEL; a bit which selects the nextaddress depending on the character and channel status, NA SEL; and fourbits which control conditional jumps based on feedback from other partsof the switching processor, SEL.

In order to facilitate the development of microcode, custom assemblershave been implemented for the queue sequencer and switching processor.These assemblers take as input an assembly code routine and outputmicrocode in the formats described above. The format for the assemblycode is similar for the switching processor and queue sequencer.Assembly statements either generate an entire microword or part of amicroword. As long as assembler output statements do not contend for thesame microcode bits, these statements may be incorporated in the samemicroword.

The addressing of memory is different for the queue sequencer and theswitching processor, reflecting the difference between their memoryformats. The memory configuration of the queue sequencer is shown inFIG. 28.

The queue sequencer's local memory holds parameters for each buffer inthe shared memory, and for each queue. This is reflected in theaddressing syntax for the memory: buffer parameters are addressed by(b.******), while queue parameters are addressed by (q.*******). Thereare three buffer parameters: the count of characters in the buffer(cnt), the successor to this buffer in a given burst (succ), and thenext buffer on the same queue as this one (next). These parameters areaddressed by the lower address bus, and are usually addressed bysymbolic constants ($cnt, $succ, $next). The buffer being examined isaddressed through the upper address bus, and may be addressed with aconstant or with the content of a register. For example, the statement:

mov-dreg=(b.-buff . $cnt ) ;

moves the count parameter of the buffer in the buff register (usuallythe current buffer for a given channel) into the data register.

Queue parameters are addressed through the middle address bus. Thissplits the upper address bit into the five most significant bits, whichaddress a particular queue by destination (output link or port), and thethree least significant bits, which address a queue by the burst type.Each queue has three parameters: a queue bottom (bot) holding theaddress of the last burst in the queue, a queue top (top) containing theaddress of the first (oldest) burst in the queue, and a queue index(indx), which keeps track of which burst types for a given destinationhave active burst on them. The index is active only in burst type 0, andso is usually addressed with a constant: (q.-queue. #0000. $indx ). Thetop and bottom parameters are usually addressed through two registers:one for the destination, and one for the burst type. For example, thestatement:

mov (q.-queue .-typ. $bot )=-buff ;

moves the contents of the buffer register into the bottom parameter ofthe burst type in the burst type register under the queue in the queueregister. Normally, this would add the current buffer to the bottom ofthe current queue. The queue parameters may also be addressed withconstants, so that the statements

mov-temp=(q. #0001 .-btyp . $bot ) ;

mov-temp=(q.-queu . #0002 . $bot ) ;

and

mov-temp=(q. $freeq. $top ) ; are valid statements.

The switching processor has a different memory address format, andconsequently the syntax of move statements for the switching processoris slightly different. The memory configuration of the switchingprocessor is shown in FIG. 29.

The switching processor's local memory is addressed by two addressbuses. The upper address bus is controlled directly from the microword,and addresses one of ten parameters. Nine of these parameters arespecific to a given channel: the status of the character coming in onthe channel (charst: whether a FLAG or DLE character has beenencountered); the status of the channel (chstat: beginning or end of aburst, etc.); the index of the next character in the active buffer(putind for LIPs and PIPs, getind for LOPs and POPs); the error controlparameter (bccsav); the routed/not-routed indicator (mark); the queue onwhich to place a new burst (outprt); and the routing table address to bechanged with the data to change it to (tabadr and tabdat). One moreaddress on the upper address bus controls the routing table entries.These entries are used to determine which communications link to route aburst onto, and are changed in the event of a link outage.

The switching processor's local memory is also addressed by the loweraddress bus. For all of the channel parameters, this is automaticallycontrolled by the channel counter. When accessing the routing table,however, the lower address bus is controlled by the index register.Thus, the instruction

mov ($charst )=-chrs ;

moves the contents of the character status register into the localmemory at the address for the character status parameter for thatparticular channel. In this way, the hardware of the switching processormay be used for other channels, while the parameters for that particularchannel are saved for use in the next frame time. On the other hand, thestatement

mov (-indx )=#0001 ;

moves the constant "1" into the routing table location addressed by theindex register. This location is independent of the channel number.Therefore, the same routing table is available to all channels as acommon resource.

Jump statements control the order of execution of microwords. This isdone by loading a value into the microaddress counter depending on aparticular condition. All jumps take two cycles to be executed, thus thestatement after a jump statement is executed whether or not the jump is"taken."

There are two forms of jump statements: normal jumps, and FIFO jumps.Normal jump statements take the branch address from the NEXTADR field inthe microword. The format of the normal jump is:

jn* $location ;

FIFO jumps take the branch address from other hardware. In the queuesequencer, the address is taken from a set of FIFOs; this results in thenext request being serviced according to a priority tree. There arethree priorities: deque, high priority, and low priority. Within apriority, requests are serviced on a first-come first-serve basis. Inthe switching processor, the address is taken from a PROM which is partof a finite state machine. The finite state machine controls thesequencing of tasks for each channel. For example, after the second byteof a burst was processed in a particular channel, the finite statemachine would indicate that the third byte was next, and control thebranching to the appropriate microcode. The format of the FIFO jumpstatement is:

jf*;

For both types of jump statements described above, the * denotes acondition symbol. If a particular condition is met, the jump isexecuted. Otherwise, the microcode address counter is incremented, andthe next (sequential) instruction is executed. Different conditions areavailable for the queue sequencer and the switching processor. For thequeue sequencer, the available conditions are:

    ______________________________________                                                   SEL                                                                           00 fall through to next instruction                                jnu        01 unconditional jump                                              jne        02 jump if equal                                                   jnn        03 jump if not equal                                               jng        04 jump if greater than                                            jnl        05 jump if less than                                               ______________________________________                                    

These are based on the result of a comparison of the contents of thedata register with some ABSOLUTE VALUE.

For the switching processor, the conditions are taken either from theALU status register or from some external hardware line. The availableconditions for the switching processor are:

    ______________________________________                                                 SEL                                                                           00 fall through to next instruction                                  jnu      01 unconditional jump                                                jne      02 jump if equal                                                     jnn      03 jump if not equal                                                 jnw      04 jump if wait flag active                                          jnb      05 jump if no buffer                                                 jns      06 jump if not start of channel                                      jnq      07 jump if queue sequencer acknowledges                              jnc      08 jump if shared memory acknowledges                                jnl      09 jump if LIP or LOP                                                ______________________________________                                    

For example, the instruction sequence:

    ______________________________________                                                  % loop jns $loop;                                                             nop;                                                                ______________________________________                                    

constitutes a loop until the start of the next channel. A nop statementrepresents a no-operation, and takes up one instruction cycle.

There are a few specialized instructions available for the queuesequencer and the switching processor. For the queue sequencer, theseinstructions are:

nst ;

pst ;

inc ;

dec ;

ein ;

din ;

The "nst" instruction strobes the output register of the normal-priorityFIFO, thus latching the next request into the output register. The "pst"instruction strobes the output register of the high priority FIFO. An"nst" must be executed by a normal priority routine, and a "pst" by ahigh priority routine, or else the contents of the FIFO output registerwill remain unchanged and the same request will be serviced many times.The "inc" instruction increments the counter, and "dec" decrements thecounter; these are used for manipulating the count parameter of abuffer. The "ein" instruction enables the addition of a burst to theindex register, while "din" enables the removal of a burst. The indexregister is used to keep track of which queues have work to do for aparticular destination; it is important to update it whenever a newburst is added to a queue or an old burst is removed.

The switching processor has special purpose instructions which arespecific to it. These are:

rst ;

bst ;

mov-outp :=-input;

alu $operation;

req $request;

The "rst" instruction is used to reset the channel strobe latch; everyswitching processor routine must do this, or miss the start of the nextchannel. The "rst" instruction strobes the input into the inputregister; for LIPs and PIPs, this latches the input from the incominglink or port; for LOPs and POPs, it requests a character from sharedmemory. Writing into the output register, though done with aconventional move statement, has a special purpose as well. For LIPs andPIPs, it initiates a write into shared memory; for LOPs and POPs, theoutput is sent to the outgoing link or port.

The "alu" instruction in the switching processor is used to control theArithmetic Logic Unit. This ALU responds to a number of codes byperforming an operation between its 15 register (-areg) and Q register(internal), and placing the result in its output register (-aluo). Theavailable ALU operations are:

    ______________________________________                                        alu     $inc;    increment areg, output to aluo                               alu     $incq;   increment areg, output to aluo and                                            qreg                                                         alu     $atoq;   transfer areg contents to qreg                               alu     $exor;   perform exclusive OR of areg & qreg                          alu     $and;    logical and of areg and qreg, output                                          to aluo                                                      alu     $andq;   logical and with output to aluo and                                           qreg                                                         alu     $decq;   decrement areg, output to aluo and                                            qreg                                                         ______________________________________                                    

The "req" instruction requests work of the queue sequencer. This is doneby sending the request along with a switching processor ID, a channelnumber, and a buffer parameter across a shared bus, and latching it intothe FIFOs of the queue sequencer. There is a request parameter for eachroutine that the queue sequencer can perform. For example, theinstruction

req $incsim ;

requests that the queue sequencer increment the count parameter of thebuffer associated with that channel of that switching processor.

The queue sequencer microcode is organized as a collection of routines,each of which services a request from a switching processor. Requestsare chosen from the highest priority FIFO which has pending work. At theend of each routine, a jfu instruction is executed, which jumps to thenext pending request. Thus, the queue sequencer executes a number ofroutines in series until there is no work to be done, at which time itexecutes a wait loop. There are presently 15 differnt routines as shownin Table 8.

                  TABLE 8                                                         ______________________________________                                        Queue Sequencer Requests                                                      Routine        Definition                                                     ______________________________________                                        incsim         simple increment count                                         incunc         increment w/getbuf                                             incunl         incunc w/enque                                                 inccon         inc, conditional getbuf                                        inccre         inc, conditional reset                                         incenq         increment and enque                                            reset          reset count                                                    requn          reset, getbuf, enque                                           resenq         reset and enque                                                deqpri         priority deque (LOPO/1)                                        deqnrm         non-priority deque=POP                                         deccon         decrement, cond. chain                                         putbuf         return buffer to freeq                                         getbuf*        get buffer for the LIP                                         setbuf*        set buffer parameters                                          ______________________________________                                         *routine is used only in initialization                                  

Only one request may be posted in any channel time from each switchingprocessor. LIPs or PIPs may post an incsim, incunc, incunl, inccon,inccre, incenq, reset, requn, or resenq request. LOPs may post a deqpri,deccon, or putbuf request, while POPs may post a deqnrm, deccon, orputbuf request. In this way, the queue sequencer is always able toprocess a request within a frame time of its posting. Deqpri has thehighest priority, those routines which enque a buffer for a LOP (incunland incenq) have the second highest priority, and all other routineshave low priority.

At the end of each routine, parameters are written into the appropriateswitching processor's dual-port buffer memory for the appropriatechannel.

Each switching processor executes a different set of microcode dependingon whether it is a LIP, LOP, PIP, or POP. The structure of the microcodeis similar for all of these. Each channel is handled independentlywithin a channel time. During this time, parameters from the localmemory are read out, requests are made to the queue sequencer,characters are inputted, processed, and outputted, and parameters arestored in local memory for the same channel in the next frame.

FIG. 30 is a functional flowchart of the input processors of a linkswitch The LIP and PIP microcode is organized as a common section ofcode which then branches to a specific routine via a jfu instruction.The common code loops until the beginning of a channel time, loads thecharacter status and channel status registers from the local memory, andstores the output of the character status finite state machine into thelocal memory. Each routine inputs a character from the communicationslink or port, does the appropriate processing, puts the character intoshared memory, requests queue handling from the queue sequencer,calculates the routine to be executed in the next frame time, and storesthis information in local memory.

FIG. 31 is a functional flowchart of the output processors of a linkswitch. The LOP and POP have somewhat simpler tasks than the LIP andPIP. The LOP need only find a burst intended for the output link or portand output the next byte of the burst. When no burst is available foroutput, a FLAG character is sent. In the case of a LOP, the timeconstraint is tight; hence the deque has highest priority. In the caseof a POP, the time constraint is not as severe and a low priority dequemay be used.

The queue sequencer handles requests from all switching processors in atwo-type priority. The first type is for processing and is based on thetype of request; the second type is for access and is based on theidentity of the switching processor.

There are three priorities in the requests: deque requests, priorityrequests, and normal requests. Priority and normal requests are servicedon a first-come first-serve basis within their type. Normal requests arenot serviced until all priority requests have been completed. The dequerequests are generated by the link output processors, and they areassigned the highest priority so that they will be serviced in the samechannel as they are made. Since deque requests are serviced in the samechannel, there is no need for the switching processor to transferidentification to the queue sequencer. Priority and normal requests areloaded into FIFOs to be serviced when the queue sequencer has time forthem. These requests must be accompanied by their IDs, which include theswitching processor number and the channel in which the request is made,so that the queue or buffer reply can be returned correctly.

The second tier of arbitration is by function of the processor. The LIP,LOP, PIP, and POP have been arranged in that order of priority. If thereare multiple processors of any type in a switch, the priority within thetype can be selected arbitrarily. The interface between the queuesequencer and the switching processors is asynchronous and prioritized.A switching processor that has the bus grant loads the request and itsID into the FIFO on the queue sequencer.

The queue sequencer is primarily a Register Transfer Machine with twoarithmetic functions (Increment and Compare) and two logic functions(AND and XNOR). There is no accumulator in the machine. Almost all bitsare linearly encoded except for the condition code select field in orderto achieve maximum speed of operation. Other than consecutiveprocessing, there are only two program control instructions: conditionaland unconditional jumps.

Both the queue sequencer and switching processor have a pipelinedinstruction register which allows automatic instruction prefetch. Theinstruction prefetch increases the performance of sequential processingand adds an additional instruction while taking a successful jump.Although it becomes necessary sometimes to insert a no-op instruction,often useful work can be done with the prefetched instruction before asuccessful jump.

After initialization, which sets all queues to empty and returns allbuffers to the free queue, the queue sequencer goes into a smalltwo-instruction loop (the second instruction is a no-op caused byinstruction prefetch) waiting for switching processor requests to comethrough the FIFO. When a request has been found, the queue sequencerjumps to the appropriate microcoded subroutine to handle the request. Atthis point, it no longer needs the request, but the identification (ID)of the requesting switching processor is required. Early in thesubroutine, a strobe is issued to the appropriate FIFO to make way forthe next request. While strobing, the current request ID is clocked intoa buffer register.

There is a local memory in the queue sequencer where it keeps the statusof the buffers and queues residing in the character memory. All burststhat go on communications links are prioritized, based on their type,into eight possible priorities (voice, data, and control being examplesof these types). There may be as many as eight link queues correspondingto eight priorities for each communications link.

Four locations are dedicated for the status of each buffer, and hencethere are two lower address bits. The buffer status contains the bytecount, next, and successor buffer linkage information. The queue statuscontains the top and bottom pointers to the queue. A priority indexregister is maintained in the zeroeth queue status of the link queues.When a LIP receives a new burst of a certain type, it sets thecorresponding priority bit in the index register not already set.Similarly, when a LOP requests the next task for an empty channel, thequeue sequencer returns the task on the highest priority queue; if thatqueue is empty, the LOP clears the corresponding bit in the indexregister.

The queue sequencer is designed to run at 12.5 MHz. The switchingprocessor is designed to run at 10 MHz.

Appendix A, entitled Burst-Switching Microcode, contains the microcodewith comments for the queue sequencer and various embodiments of theswitching processor, e.g., LIP, LOP, PIP, POP, etc.

PORT CIRCUIT

The purpose of a port circuit or port-interface circuit in aburst-switching system is to transform the signal form of thelink-switch port to the particular signal form of the port user, andconversely. Thus, analog telephones, digital telephones, data devices,analog trunks, etc., each will require its own type of port circuitwherein the characteristics of its signal are transformed to the commonburst port-signal form. While port circuits for various types of deviceswill differ, all will include processing means for creating andterminating bursts, silence/voice detection, and other controlfunctions.

In the following, the port circuit for an analog telephone is described.Thereafter, the relationships of other types of port circuits to theanalog-telephone port circuit will be described.

In FIGS. 3 and 6, port circuits 178 and 258 respectively are shown asbeing coupled with ports of link switch 132. FIG. 32 shows a blockdiagram of an example of a port circuit 950 for an analog line which maybe employed as either port circuit 178 or port circuit 258.

In FIG. 32, some of the so-called BORSCHT functions are contained inport circuit 950. This is consistent with the highly distributed controlfeature of a burst switch. The term "BORSCHT" is a mnemonic whichrepresents the standard functions traditionally associated with a linecard in a digital switching system. These functions are as follows. "B",is for Battery feed: supplies the direct-current bias or loop current tothe subscriber instrument; "O", is for Overvoltage protection: protectsagainst damage by high-voltage transients, such as those induced bylightening strikes near the transmission line; "R", is for Ringing:controls the ringing signal induced on the subscriber line; "S", is forSupervision: monitors the line to detect various subscriber instrumentconditions; "C", is for coding: codes the subscriber's voice signal intodigital characters and conversely; "H", is for Hybrid: performs thetwo-to-four wire conversion required between the two-wire subscriberline and the four-wire coding section; and "T", is for Test: performstests, e.g., on the subscriber line, to determine fault conditions. Fora more detailed explanation of the BORSCHT functions, see Designer'sHandbook of Integrated Circuits, Arthur B. Williams, Editor in Chief,McGraw-Hill Book Company, 1984, Chapter 4.

Bursts of every type from port circuit 950 pass byte-by-byte between theport bus and microprocessor 952 through interface 954. Interface 954matches the characteristics of the port bus to the bus of microprocessor952. A commercially available microprocessor, such as the Zilog Z80A,will suffice as microprocessor 952.

The bytes of an incoming voice burst arriving at microprocessor 952 willbe delivered to Digital-to-Analog (D/A) converter 956. D/A converter 956produces a continuous analog signal which is transmitted to SLIC 958.

Subscriber Line Interface Circuit (SLIC) 958 is a commercially availablecircuit which performs many of the BORSCHT functions, including thehybrid or four-to-two wire conversion function, battery feed or poweringof the telephone, application of ring voltage to the line, and off-hookdetection. The reformulated analog signal is applied to the two-wireanalog line 960 by SLIC 958.

The analog signal arriving from the end-user instrument, e.g., atelephone, on analog line 960 passes through SLIC 958 and is applied tothe Analog-to-Digital (A/D) converter 962. A/D converter 962 digitizesthe signal and provides a character stream to microprocessor 952.Microprocessor 952 continuously executes a voice/silence detectionalgorithm against this stream. If microprocessor 952 determines thatinformation energy is present, e.g., the user is speaking, it declares aburst, prefixes a header, sends the bytes of the burst via interface 954to the port bus, and appends a terminator or termination sequence at theend of the burst. The D/A and A/D converters, taken together, are oftenreferred to as the "codec," for coder/decoder. Thus, burst originationand termination occur in the port circuit. This feature is of particularimportance when the port circuit is moved from the vicinity of the linkswitch to the vicinity of the user, ultimately within the end-userinstrument itself. Burst-switching architecture supports digitaltransmissions from origin-user to destination-user including a digitaltelephone.

Voice/silence detection algorithms are known in the art. Since thedetection algorithm operates on a digital signal, digital speechinterpolation (DSI) algorithms are appropriate. For an example of a DSIalgorithm, see S. J. Campanella, Digital Speech Interpolation, ComsatTech. Rev., Vol. 6, No. 1, pp. 127-158, Spring, 1976. Also, equivalentTASI (Time Assignment Speech Interpolation) techniques known in the artare applicable.

When certain conditions arise in port circuit 950, microprocessor 952creates a control burst and sends it through interface 954 to informhigher level processors of these conditions. Such conditions includeon-hook or off-hook detection, and tone detection from tone detector964. Upon receipt of an analog tone, such as a key tone, tone detector964 supplies a digital encoding of that tone to microprocessor 952. Anexample of a suitable tone detector is GTE Microcircuits, Part No.G8870A.

When microprocessor 952 receives a control burst from interface 954, itmay take various actions depending upon the control burst type. It mayturn on ringer 966 which generates a 20 Hertz ring voltage which causinga telephone attached to the line to ring. It may cause a sequence ofbytes representing an analog tone to be read from the memory ofmicroprocessor 952 to be sent to D/A converter 956 which will cause atone, e.g., a dial tone or a busy signal, to be heard at the telephone.The control burst may also cause the D/A output to be connected directlyto the A/D input through loop-back circuit 968 which enablescomprehensive remote diagnostic capability in a burst-switching system.

Looping the port circuit back provides means for a comprehensive testmode. A voice burst received from interface 954 will be reflected backas a voice burst to the interface, having been routed through thefollowing circuit components: interface 954, microprocessor 952, D/Aconverter 956, loop-back 968, A/D converter 962, microprocessor 952, andinterface 954. The voice burst received by a higher level processor maybe compared (digitally) with the one sent, and the operability of thesecomponents determined. Also, tones received by tone detector 964 may becompared to tones sent from microprocessor 952, so that the operabilityof tone detector 964 may be monitored.

This testing capability leaves analog line 960 untested. Port circuit950 can be moved from the link switch to the vicinity of the analog endinstrument, if port bus interface 954 is replaced by a digital-lineinterface. For example, if port circuit 178 of FIG. 3 is physicallylocated near an analog end instrument, then port circuit 950 performsthe functions of port circuit 258 shown in FIG. 6. The line to thevicinity of the subscriber (perhaps on the subscriber premises or evenwithin the end-user equipment) becomes a digital line, and the entireline including the remote port circuit can be tested using loop-backcircuit 968.

Port circuit 950 will serve a digital voice telephone if the hybrid andbattery feed circuit within SLIC 958 is removed. The output of D/Aconverter 956 goes directly to the telephone earpiece, and the telephonemike output goes directly to A/D converter 952.

A port circuit suitable for a digital terminal would comprise interface954 and microprocessor 952 of port circuit 950, plus a commerciallyavailable parallel-to-serial and serial-to-parallel converter called aUART (Universal Asynchronous Receiver Transmitter).

Port circuit 950 performs the functions of an existing artcircuit-switched line circuit, which consists of the codec (D/A and A/Dconverters 956 and 962) and SLIC 958.

Port circuit 950 differs from existing art line circuits in at least thefollowing respects:

1. It includes interface 954 which permits use of a digital line andremote placement of the port circuit.

2. It includes microprocessor 950 which permits interpretation ofcontrol bursts and silence/voice detection.

3. It includes a local ringer circuit 966 which permits local generationof ring voltage rather than the common generation of ring voltage formany line circuits.

4. It includes tone detector 964 which permits local detection of signaltones rather than relying on a common tone detector shared across manyline circuits.

5. It includes loop-back circuit 968 which permits remote testing of allof the port circuit except for SLIC 958.

DISTRIBUTED CONTROL

FIG. 1 shows burst-switching system 100 comprising a network of linkswitches interconnected by links. Bursts may enter and leave the networkvia ports coupled with end users. In system 100, suppose user X desiresto communicate with user Y. A burst entering the network at X's portwith Y's address in its header must be routed by the network to Y'sport. This routing requires that each link switch have knowledge of thenetwork, so that a link switch can transmit the burst toward itsdestination. More specifically, each link switch must have sufficientintelligence to determine from the burst header the following:

1. If the destination is not local to itself, which link leads mostdirectly toward the destination port. In general, more than one link isconnected to a link switch.

2. If the destination is local to itself, to which port should the burstbe delivered.

The burst switch control comprises a set of microprocessors eachappearing at ports of the network. Each control processor sends andreceives messages called control bursts. There are three functionallydistinct types of processors in the control port processors, callprocessors, and administrative processors.

As has been described above, each system port has a port processorsituated in the port circuit, e.g., a line circuit. A port processor mayexchange control bursts with its call processor. A port processor maydetect external signals, such as off-hook, tones, etc., in conjunctionwith other circuits in the port circuit; in response, the port processorsends control bursts. A port processor may receive control bursts fromother control processors; in response, the port processor sends externalsignals, such as ring, tones, etc., in conjunction with other circuitsin the port circuit. The nature of the external signals will varydepending upon the type of port circuit. In this way, a port processoracts as a converter between external signals and internal controlbursts.

Each port circuit has a port processor. Since it is generally desirableto keep the port processor's memory small for economy reasons, the portprocessor program is not large. Variations in the kind of port, e.g.,line, trunk, etc., are handled in the port processor so that the controlburst interface is nearly the same for all types of ports.

The bulk of the higher-level logical functions in a burst-switchingsystem is located in the call processors. Call processors handle callset-up, custom-calling feature execution, various maintenance tasks,etc. As a consequence of the diversity of tasks, the program memory ofcall processors may be quite large.

Each call processor has a port appearance. That is, the call processorappears to the switching network as if it were a data device. Itsnetwork appearance is that of a user computer. Yet, it is directlyinvolved in the control of the switch itself, in ways that will bedescribed below. A switch control generally will include a number ofcall processors: as many as the call-handling load, availability, andsurvivability of the switch require. A currently availablemicroprocessor, such as a Motorola 68000, will provide ample processingcapacity for the call processor.

A burst-switching system necessarily includes a number of administrativeprocesses. For example, in a telephone system, typical administrativeprocesses are directory-number to equipment-number translation, time andusage accumulation, recent changes, maintenance, etc. These processesare mainly database activities. Thus, administrative processes arecharacterized by large data-storage and moderate program-storagerequirements.

In smaller systems, the administrative processes might be performed bythe call processors. In larger systems, they likely will be performed byseparate administrative processors. In larger applications, theadministrative processors may comprise call processor hardware withlarge storage capability, requiring only a port interface as for callprocessors proper. Thus, a port of a link switch may be coupled with auser line or with a trunk to another communications system, thesecouplings including a port processor; or the port may be coupled with acall processor or administrative processor.

A burst-switch control will include as many administrative processors asthe administrative-processing load, availability, and survivability ofthe system require.

As described, control comprises a port processor for each user port,some number of call processors, and some number of administrativeprocessors. In the following, the discussion addresses how theseelements perform the control function. The unifying principle is that ofprovision of service. All processors ultimately perform services forusers. Port processors perform services for users directly; callprocessors perform services for port processors; and administrativeprocessors perform services for call processors. In order to formalizethe discipline between the various control processors, it is useful tointroduce the notion of a "service set" associated with each processor.To this end, the following definitions apply:

service processor: a processor providing services to others;

service set: all for whom a processor performs services;

service set member: one for whom a processor performs services; and

service set head: the processor which performs services for a serviceset.

FIG. 33 illustrates the notion of a service set and the hierarchy ofservice providers; note that:

each port processor has one user in its service set;

each call processor has a number of port processors in its service set;and

each administrative processor has a number of call processors in itsservice set.

In FIG. 33, four groups of users are shown, G₁, G₂, G₃, and G₄, by wayof example. Each group may contain a reasonable number of users, notnecessarily two as depicted in the drawing for simplicity. Each user isthe service set for the respective port processor (PP) with which it iscoupled. Each group of port processors comprises the service set (SS)for the call processor (CP) with which the group is coupled. Each groupof call processors comprises the service set for the administrativeprocessor (AP) with which the group is coupled. Thus, USER₅ is servicedby PP₅ ; PP₅, being a member of SS₃, is serviced by CP₃ ; and CP₃, beinga member of SS₆, is serviced by AP₂.

FIG. 33 suggests that there are more port processors than callprocessors, and more call processors than administrative processors.This is generally the case. Since port processors perform silencedetection on voice ports, they are expected to be rather busy, even inserving a single port. Typically, a port initiates a call onlyoccasionally, so that a number of port processors may be served by asingle call processor. Because the program memory of a call processor isexpected to be significant, there is a cost advantage in providing onlythe required number of call processors.

The administrative processor involvement in a call is small, even lessthan that of a call processor, so that fewer administrative processorsare required than call processors. Since the database memory requirementof an administrative processor is expected to be significant, there is acost advantage in providing only the minimum number of administrativeprocessors.

Although FIG. 33 implies a hierarchy of service sets, it should beemphasized that all processors operate autonomously. The illustration inFIG. 33 is not intended to imply that a processor block may in somesense control a processor block to its left. Instead, the hierarchy isintended to show service relationships, with requests for serviceflowing to the right and responses to such requests flowing to the left.

As noted above, smaller systems may not require separate administrativeprocessors in which case administrative processes will be executed bycall processors. In fact, there is nothing in the architecture whichnecessarily requires call processors. All processes could operate at theport level. This would require substantial memory in every portprocessor. The relationships of FIG. 33 permit a system to implement thecontrol functions in an efficient manner with shared and distributedresources.

In some embodiments of a burst-switching system, particularly those withstringent survivability requirements, port circuit intelligenceincluding control may be located on user premises or even withinend-user instruments. In such embodiments, all that is necessary forcommunications to occur between two (or more) surviving users is themutual availability of communications--transport means, e.g., a linkswitch coupled between surviving users.

Members of a service set may be located anywhere in the switchingnetwork. There is no requirement that they be close together,contiguous, etc. As a practical matter, however, the farther a controlmessage must travel the more network resources are employed in itstransmission. In order to minimize network resources dedicated tocontrol, it is reasonable to expect the members of a service set to benear one another with the head of the service set being located near thecenter of the set.

Control-burst traffic between a port processor and its call processorrequires that each processor possess the network address of the other.As a "connection" is defined in these terms, another definition of aservice set is that there is a connection between each member of the setand the set's head. As with all burst connections, no network resourcesare dedicated by these connections.

Suppose in FIG. 1, user X desires to communicate by telephone with userY. FIG. 34 shows the typical burst traffic between control processors inorder to set-up and terminate a simple call.

In step 1, X's off-hook signal is detected by X's port processor(PP_(X)). PP_(X) sends an appropriate control burst to X's callprocessor (CP_(X)).

In step 2, CP_(X) sends a control burst to PP_(X) which causes PP_(X) tosend a dial tone to X. Hearing the dial tone, X commences dialing Y'stelephone number.

In step 3, PP_(X) detects the first digit dialed by X. PP_(X) sends thisdigit to CP_(X) in a control burst. This process continues digit bydigit.

In step 4, PP_(X) detects the last digit dialed by X. PP_(X) sends thisdigit to CP_(X) in a control burst.

In step 5, CP_(X) sends Y's directory telephone number (dialed by X) andthe equipment address of CP_(X) to X's administrative processor (AP_(X))in a control burst. AP_(X) looks up the equipment address of Y's callprocessor (CP_(Y)) and Y's port processor (PP_(Y)).

In step 6, AP_(X) sends the equipment addresses for CP_(Y) and PP_(Y) toCP_(X) in a control burst.

In step 7, CP_(X) sends a control burst to CP_(Y) inquiring whetherPP_(Y) is idle. This control burst includes CP_(X) 's and PP_(X) 'sequipment addresses.

In step 8, CP_(Y) responds to CP_(X) by sending a control burstindicating whether or not PP_(Y) is idle. (If PP_(Y) is not idle, CP_(Y)sends a control burst to CP_(X) which will cause a busy-signal tone tobe applied to X's line. This contingency is not shown in FIG. 34.) Inthe example of FIG. 34, it is assumed that PP_(Y) is idle.

In step 9, CP_(X) and CP_(Y) each send control bursts to PP_(X) andPP_(Y) respectively. The control burst to PP_(Y) includes PP_(X) 'sequipment address and causes PP_(Y) to initiate ringing of Y's phone.The control burst to PP_(X) includes PP_(Y) 's equipment address andcauses PP_(X) to initiate a ring-back signal on X's phone. At thispoint, both port processors know the network address of the other party.

In step 10, Y lifts his handset. PP_(Y) detects Y's off-hook conditionand sends a control burst to CP_(Y) indicating this condition.

In step 11, CP_(Y) sends a control burst to CP_(X) indicating Y'soff-hook condition.

In step 12, CP_(X) sends a control burst to PP_(X) which causes PP_(X)to terminate the ring-back signal on X's line.

Thereafter, a full-duplex conversation between X and Y ensues, using thepreviously sent equipment addresses of the parties. Bursts originatingfrom PP_(X) are sent directly to PP_(Y) without administrative overhead,and likewise for bursts from PP_(Y) to PP_(X). The port processor ofeach party knows the network address of the port processor of the otherparty.

In step 13, PP_(Y) detects that Y has hung up. PP_(Y) sends a controlburst to CP_(Y) indicating Y's on-hook condition and including usageinformation.

In step 14, PP_(X) detects X's on-hook condition. PP_(X) sends a controlburst to CP_(X) indicating X's on-hook status and including usageinformation.

In step 15, CP_(X) sends a control burst to AP_(X) indicating completionof the call and including time and usage information for billing and/oradministrative purposes.

In step 16, AP_(X) sends a control burst to CP_(X) acknowledging receiptof the control burst sent in step 15.

FIG. 35 illustrates some of the control bursts in the method of callset-up and take-down shown in FIG. 34. In the drawing showing a portionof system 100, control bursts are shown as dashed lines between controlprocessors for illustration purposes. All control bursts are transmittedthrough the network in the same manner as other types of bursts. Thenumbered steps in the drawing correspond to the numbered steps describedabove. The heavy dashed line between PP_(X) and PP_(Y) indicates thevoice conversation (two-way) between X and Y.

FIG. 35 also depicts a typical control architecture for system 100.CP_(X) is the call processor for a number of port processors includingPP_(X). Likewise, CP_(Y) is the call processor servicing Y and a numberof other users. AP_(X) is the administrative processor for a number ofcall processors including CP_(X) ; AP_(Y) is the call processor forCP_(Y). AP_(Y) has no responsibilities in this embodiment of the callset-up and take-down method because X is the originator of the call. Inalternative embodiments of the method, particularly where customfeatures are implemented, AP_(Y) may play a role in call set-up andtake-down. Although X and Y are users of different link groups and areshown in the drawing as being in different service sets of therespective control processors, there is nothing in the architecture thatrequires this arrangement. For example, CP_(Y) might be in APX's serviceset; and/or it may be that CP_(X) services both PP_(X) and PP_(Y). Thereis no requirement at any level of control that members of a service setbe restricted to one (or more) link groups.

As outlined in FIG. 34, a call may be set-up and taken down in aburst-switching system with highly distributed control architecture. The"highest" level of control required is that of the administrativeprocessor which, rather than being situated in a central office, iscoupled with a port of a conveniently located link switch or hub switch.The control traffic utilizes the network itself for transport. If X andY were local to each other, the control necessary to effectuate the callwould be no farther away than the locations of the control processorsservicing X's line. As has been stated above, this control architecturemay be employed for implementing custom features in addition to regularcall services.

A control processor may be added to a service set by sending the networkaddress of the set's head to the control processor. Thereafter, thecontrol processor will forward its service requests to its serviceprocessor, the service set's head. There is no requirement that theservice-set head be the sender of the assignment message, although thatwill frequently be the case. In the general case, a first controlprocessor may assign a second control processor to the service set of athird control processor.

Referring to FIG. 33, CP₁ adds PP₁ to its service set by sending its(CP₁ 's) address to PP₁ in a control burst; AP₁ adds CP₁ to its serviceset by sending its (AP₁ 's) address to CP₁ in a control burst. In FIG.34, PP_(X) possesses CP_(X) 's address for control burst communicationsin a call set-up.

Establishment of service sets is easy because it may be accomplished bysending of control bursts between processors of the control. Servicesets can be redefined equally easily.

In the event a control processor should fail, there must be means forreassigning members of the service set of the failed processor to theservice set of an alternate control processor. This can be accomplishedeasily within the network by sending service set reassignments incontrol bursts.

In FIG. 33, if CP₁ should fail, it is necessary to reassign portprocessors PP₁ through PP₂ (generally there will be many more than twoport processors in a group) to other service sets. Perhaps the simplestway to do this is to assign them to CP₂ 's service set by sending eachof them CP₂ 's address. This may approximately double CP₂ 's load. Amore uniform redistribution might be better. For example, suppose asystem has N equally loaded or approximately equally loaded callprocessors and one fails. Each of the remaining N-1 call processorsmight pick up 1/(N-1) of the port processors in the failed callprocessor's service set. In this case, each control processor's loadwould increase only by a factor of N/(N-1). Another approach may be toincorporate into a control processor emergency capacity to substitutefor another processor.

An important feature is the ease with which control capacity may beadded to an existing burst-switching system. Suppose a system has Nequally loaded or approximately equally loaded call processors, andadditional control capacity is desired. If a new call processor isinstalled at an idle port and if 1/(N+1) of the port processors arereassigned to the service set of the new call processor, the resultingsystem has N+1 call processors. If the reassigned port processors aredrawn uniformly from the original N service sets, the resultant systemhas N+1 equally loaded or approximately equally loaded call processorswherein each of the original call processors has its workload reducedcommensurately, i.e., by a factor of 1/(N+1). Thus, additional capacitymay be installed into the system incrementally and with virtually nodisruption in user services.

When a control processor has been installed in a burst-switching system,software installation may be performed within the network itself. Thesoftware for the particular processor may be downloaded through thesystem; initialization and start-up testing likewise may be performedthrough the network. Future software modifications for the processor, orindeed any processor, may be installed automatically through thenetwork. This start-up capability within the network is one aspect ofthe self-contained maintenance and administrative capability of a burstswitch which provides flexible management of system resources with fastresponse and minimal cost.

However control reassignment may be accomplished, two points aresignificant with respect to the architecture of a burst-switchingsystem. First, no special switchover network is required. Reassignmentis accomplished entirely through the existing network. Second,allocation of redundant control capacity is extremely flexible andefficient, unlike the one-for-one redundancy often found in currentduplex systems.

It is necessary to determine that a processor has failed beforeswitchover can occur. In burst-switching control, processor failure isdetermined by lack of response to test message traffic. The redefinitionof service sets is coordinated by the administrative processors workingwith the surviving call processors.

The burst-switching control is a multiprocessor system because itperforms the control task using many processors. It differs from theusual multiprocessor system. A typical multi-processor system providestight coupling among processors using common buses or shared memory;such architecture is itself subject to failure and must be duplexed. Ina burst-switching network, the multiprocessor system is created fromprocessors which are loosely coupled by message exchange through thenetwork. This loose coupling provides substantially greater flexibilityand economy in the management of system resources and in ease ofexpansion. Capacity can be expanded by the addition of processors toidle ports. The added processors can be brought into service easily byredefinition of the service sets, whether the new processors are call oradministration processors.

Because the multiprocessor control processors are coupled only via theswitching network, there are no processor intercommunications buses orcommon memories. Redundant buses and common memories are not required.Switchover circuits between one bus and the other, or between one commonmemory and the other, are not required. Sensing or comparison circuitry,to determine if failure has occurred, is not required. For thesereasons, it is believed that maintenance will be less complicated thanwould be required in a typical one-to-one redundancy system.

Burst-switching control provides an architecture different from thecontrol architecture of the central office and PBX circuit switches oftoday. The burst-switching distributed-control architecture offers thefollowing advantages:

1. Easy capacity redistribution and expansion: the system will not beimmobilized by the failure of a single (or several) control processors;in the event a control processor should fail, the remaining controlprocessors will substitute for the failed processor so that userservices will be maintained; control capacity may be added to the systemincrementally with virtually no disruption in user services;

2. Easy and efficient processor failure/recovery using the switchingnetwork itself, without standby processors for each active processor,switchover networks, and error-detecting comparitors;

3. Expected reduction in the complexity of the maintenance functionbecause of the simplified architecture;

4. Independent operation of call processors so that a burst-switchingsystem may concurrently originate as many calls as there are portprocessors in the system, which translates into the economic advantagethat control may be added to the system incrementally; and

5. More economical use of system resources because of the previousadvantages.

While there have been shown what are at present considered to bepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madeherein without departing from the scope of the invention as defined bythe claims which appear after the following appendix to thespecification. ##SPC1##

We claim:
 1. A link switch for a burst-switching communications system,a burst being a plurality of bytes including a destination-port address,an information portion, and a termination portion at the end of theburst, said system including a plurality of switches interconnected bytime-divided communications links, each link having a plurality offrames within each second of time, each frame having a plurality ofchannels, each channel having communications capacity for thetransmission of one byte, a byte being a predetermined number of bits, abit being one binary digit, said link switch comprising:(a) a centralmemory; (b) link-input processing means coupled with said centralmemory, said link-input processing means having means for coupling witha communications link, means for receiving bursts on said communicationslink, and means for accessing said central memory; (c) link-outputprocessing means coupled with said central memory, said link-outputprocessing means having means for coupling with a communications link,means for transmitting bursts on said communications link, and means foraccessing said central memory; (d) at least one port, each portproviding means for coupling with a control processor, an end-userinstrument, or another communications system, said coupling being madevia a port-interface circuit; (e) port-input processing means coupledbetween said central memory and at least one of said ports, saidport-input processing means having means for receiving bursts from saidport(s) and means for accessing said central memory; (f) port-outputprocessing means coupled between said central memory and at least one ofsaid ports, said port-output processing means having means fortransmitting bursts to said port(s) and means for accessing said centralmemory; (g) memory-management means coupled with said central memory forinsuring that at any given time only a single access to said centralmemory may occur; (h) means for routing a burst through said switch,said routing means having processing capacity sufficient to support aneffective transmission rate of at least 1.544 millioninformation-bearing bits per second on each of said communications linkscoupled with said link switch, said routing means including:(i) meansfor routing a burst through said switch from an origin port of saidswitch to a destination port of said same switch; (ii) means for routinga burst through said switch from an origin port of said switch to a linkof said switch leading toward the destination port; (iii) means forrouting a burst through said switch from a link leading into said switchto a destination port of said switch; and (iv) means for routing a burstthrough said switch from a link leading into said switch to a link ofsaid switch leading toward the destination port; and (i) means forrouting from port to link and from link to link employing dynamicallocation of bursts to channels within links such that any channel isallocated only when a burst is being transmitted therein and saidchannel is otherwise available for transmission of another burst, saidswitch after receiving the first byte of a burst and also determiningthat said burst will be transmitted on a link of said switch leadingtoward the destination port then transmitting said first byte in thefirst available channel of a first frame of said link leading towardsaid destination port and assigning said channel to said burst, saidswitch after receiving the second or a successive byte of said burstthen transmitting said second or successive byte in said assignedchannel of the second or successive frame of said link, said switchafter transmitting the last byte of said burst then releasing saidassigned channel for possible allocation to another burst in the frameimmediately following the transmission of the last byte of said burst.2. A link switch for a burst-switching communications system asdescribed in claim 1 wherein said switch includes a port bus, and saidports of said switch are sequentially coupled on said port bus, saidport bus being time-divided and having a time interval associated witheach port on said port bus.
 3. A link switch for a burst-switchingcommunications system as described in claim 1 wherein said switchincludes a multiplexer circuit, and said ports are coupled with saidmultiplexer circuit.
 4. A link switch for a burst-switchingcommunications system as described in claim 1 wherein said switchincludes at least one port-interface circuit, said port-interfacecircuit being coupled with at least one of said ports.
 5. A link switchfor a burst-switching communications system as described in claim 4wherein at least one of said port-interface circuits is a line circuithaving means for coupling with an end-user instrument.
 6. A link-switchfor a burst-switching communications system as described in claim 4wherein said port-interface circuit includes a port processor havingmeans for implementing control functions.
 7. A link switch for aburst-switching communications system as described in claim 4 whereinsaid port-interface circuit is a trunk circuit having means for couplingwith another communications system.
 8. A link switch for aburst-switching communications system as described in claim 1 whereinsaid byte is eight bits.
 9. A link switch for a burst-switchingcommunications system as described in claim 8 wherein saidcommunications link is a T-carrier link.
 10. A link switch for aburst-switching communications system as described in claim 1 whereinthe transmission rates received at said ports for bursts containingdigitized speech are approximately equal to burst transmission ratesover communications links, so that speed buffering of speech burstswithin said switch is not required.
 11. A link switch for aburst-switching communications system as described in claim 8 whereineach burst includes at least one preassigned flag character in thetermination portion of the burst, and wherein a preassigned data-linkescape character is inserted immediately preceding any data byte of aburst which has the same bit configuration as either said flag or saiddata-link escape character, whereby said system may distinguish betweena data byte having the same bit configuration as a flag character andthe flag character as a terminator.
 12. A link switch for aburst-switching communications system as described in claim 11 whereinsaid flag and data-link escape characters have preassigned bitconfigurations equal to the digital codec outputs representing themaximum positive and maximum negative amplitudes of the analog inputsignal, respectively or conversely.
 13. A link switch for aburst-switching communications system as described in claim 11 whereinsaid flag and data-link escape characters have preassigned bitconfigurations equal to the digital codec outputs representing theminimum positive and minimum negative amplitudes of the analog inputsignal, respectively or conversely.
 14. A link switch for aburst-switching communications system as described in claim 8 whereinsaid burst includes a variable-length information portion, a four-byteheader preceding said information portion, and a one-byte terminatorfollowing said information portion, said header including saiddestination-port address.
 15. A link switch for a burst-switchingcommunications system as described in claim 1 wherein a burst will betransmitted over a communications link leading away from said switchonly when the destination port of said burst is a component of a switchother than said switch.
 16. A link switch for a burst-switchingcommunications system as described in claim 1 wherein the number ofports of said link switch is less than or equal to thirty-two.
 17. Alink switch for a burst-switching communications system as described inclaim 1 wherein said switch has a plurality of communications linkscoupled therewith.
 18. A link switch for a burst-switchingcommunications system as described in claim 1 wherein said link-inputprocessing means, said link-output processing means, said port-inputprocessing means, and said port-output processing means each includes afirmware or software variant of a switching processor.
 19. A link switchfor a burst-switching communications system as described in claim 18wherein said central memory comprises a character memory for storingcharacters and queues of characters, and a queue memory for storingadministrative and control information pertaining to said characters andqueues stored in said character memory.
 20. A link switch for aburst-switching communications system as described in claim 19 whereinsaid link switch further comprises a queue sequencer for administeringsaid information in said queue memory, said queue sequencer having theability to act on behalf of and in response to each of said switchingprocessors, said queue sequencer having the ability to operatesubstantially in parallel with any of said switching processors.
 21. Alink switch for a burst-switching communications system, a burst being aplurality of bytes including a destination-port address, an informationportion, and a termination portion at the end of the burst, a byte beinga predetermined number of bits, a bit being a binary digit, said linkswitch comprising:(a) a central memory; (b) a plurality of ports, eachport providing means for coupling with a control processor, an end-userinstrument, or another communications system, said coupling being madevia a port-interface circuit (c) port-input processing means coupledbetween said central memory and at least one of said ports, saidport-input processing means having means for receiving bursts from saidport(s) and means for accessing said central memory; (d) port-outputprocessing means coupled between said central memory and at least one ofsaid ports, said port-output processing means having means fortransmitting bursts to said port(s) and means for accessing said centralmemory; (e) memory-management means coupled with said central memory forinsuring that at any given time only a single access to said mmeory mayoccur; and (f) means for routing a burst through said switch from anorigin port of said said switch to a destination port of said sameswitch, said routing means having processing capacity sufficient tosupport an effective transmission rate of at least 1.544 millioninformation-bearing bits per second on each port of said plurality ofports coupled with said link switch.
 22. A link switch for aburst-switching communications system as described in claim 21 whereinsaid switch includes a port bus, and said ports of said switch aresequentially coupled on said port bus, said port bus being time-dividedand having a time interval associated with each port on said port bus.23. A link switch for a burst-switching communications system asdescribed in claim 21 wherein said switch includes a multiplexercircuit, and said port(s) are coupled with said multiplexer circuit. 24.A link switch for a burst-switching communications system as describedin claim 21 wherein said switch includes at least one port-interfacecircuit, said port-interface circuit being coupled with at least one ofsaid ports.
 25. A link switch for a burst-switching communicationssystem as described in claim 24 wherein at least one of saidport-interface circuits is a line circuit having means for coupling withan end-user instrument.
 26. A link-switch for a burst-switchingcommunications system as described in claim 24 wherein saidport-interface circuit includes a port processor having means forimplementing control functions.
 27. A link switch for a burst-switchingcommunications system as described in claim 24 wherein saidport-interface circuit is a trunk circuit having means for coupling withanother communications system.
 28. A link switch for a burst-switchingcommunications system as described in claim 21 wherein said byte iseight bits.
 29. A link switch for a burst-switching communicationssystem as described in claim 28 wherein each burst includes at least onepreassigned flag character in the termination portion of the burst, andwherein a preassigned data-link escape character is inserted immediatelypreceding any data byte of a burst which has the same bit configurationas either said flag or said data-link escape character, whereby saidsystem may distinguish between a data byte having the same bitconfiguration as a flag character and the flag character as aterminator.
 30. A link switch for a burst-switching communicationssystem as described in claim 29 wherein said flag and data-link escapecharacters have preassigned bit configurations equal to the digitalcodec outputs representing the maximum positive and maximum negativeamplitudes of the analog input signal, respectively or conversely.
 31. Alink switch for a burst-switching communications system as described inclaim 30 wherein said flag and data-link escape characters havepreassigned bit configurations equal to the digital codec outputsrepresenting the minimum positive and minimum negative amplitudes of theanalog input signal, respectively or conversely.
 32. A link switch for aburst-switching communications system as described in claim 28 whereinsaid burst includes a variable-length information portion, a four-byteheader preceding said information portion, and a one-byte terminatorfollowing said information portion, said header including saiddestination-port address.
 33. A link switch for a burst-switchingcommunications system as described in claim 21 wherein the number ofports of said link switch is less than or equal to thirty-two.
 34. Alink switch for a burst-switching communications system as described inclaim 21 wherein said port-input processing means and said port-outputprocessing means each includes a firmware or software variant of aswitching processor.
 35. A link switch for a burst-switchingcommunications system as described in claim 34 wherein said centralmemory comprises a character memory for storing characters and queues ofcharacters, and a queue memory for storing administrative and controlinformation pertaining to said characters and queues stored in saidcharacter memory.
 36. A link switch for a burst-switching communicationssystem as described in claim 35 wherein said link switch furthercomprises a queue sequencer for administering said information in saidqueue memory, said queue sequencer having the ability to act on behalfof and in response to each of said switching processors, said queuesequencer having the ability to operate substantially in parallel withany of said switching processors.